Because I can, here's a skills guide!
While most any strategy has some chance of making it to the top in Life of a Mobster, some skills really are more useful than others in almost every path. Simply put, you're going to get more benefit out of Persuasion than you'll ever get out of Medical with almost any concept.
When a skill is said to be "useful" for an ending, that means directly useful in the endgame, except in the case of the hitman ending, whose skill requirements are met by remaining a hitman at the end of your Capo period.
Tier 1: These skills are vital for everyone. You want one of them if you're going anywhere, and you really want both. (All right, I'll amend that. A pure hitman can prosper without relying on either, if he's got enough gun skills to make up for his lack of brains.) Without either of them, it's going to be difficult (though not impossible) to get a good ending.
Persuasion: Smooth criminal. This is useful at all levels, but it's especially good at keeping the heat off you in the early game and building up rep with citizens and criminals alike. Persuasion is absolutely required for a career in politics, which is a useful edge in a lot of places, but you only need a moderate ability with it for that if you have Subterfuge. Useful for the Good Life, Hollywood, G-Man and Suburbia endings (though you'll get the Informant ending anyway, you want to take all the Families down, don'cha?). Also useful for avoiding the Big Bang.
Intimidation: This is the skill for the hard rock, and it gets a lot of use too, especially in the early game, where it's excellent for getting stuff done and kickstarting your early earning (and in the early game, even a few hundreds can make a difference). Beware, though: muscling people attracts a hell of a lot of heat in the early game, which can cost you a few benefits. Intimidation is also the best way to get the mob friend on your side, along with a solid criminal rep. Useful for the Good Life ending.
Tier 2: These skills are useful for everyone in some places, but you don't need all of them. Still, look into getting 'em.
Firearms: I'm putting Firearms here because of the Pacifist achievement; if you're consciously staying away from the wet work, then your gun skill is useless. If you're not, it's almost as useful as Persuasion and Intimidation. Just about every serious problem can be solved with enough bullets, and being a hitman is highly lucrative. Also a productive way to deal with surviving bosses if you're going for the top spot. Useful for the Hitman and Boss of Bosses endings.
Subterfuge: If you're a politician, this is almost Tier 1; except at the first election, it's a completely equal replacement for Persuasion, and it's also necessary to stay as a politician and get involved in the counterfeiting business. Elsewhere, it's very useful for pacifists and hitmen alike, and also leads to one of the better solutions to the Jane Garcia subplot. Useful for the Got Out ending and for avoiding the Big Bang.
Perception: Used in a lot of different places to pinch-hit for just about any skill, circumstantially. A modest level of this is vital for a hitman, and it's also important to have if you need to hold out on Harcourt. It's checked for nearly as many things as Persuasion, but it's much easier to cover for a lack of Perception than a lack of Persuasion. Useful for the Hitman, Got Out and Hollywood endings, and for avoiding the Big Bang.
Athletics: Ordinarily, this would be a Tier 3 skill, except that it's vital for completing the Hitman sub-story (and thus the Hitman ending), and can earn a fair chunk of money from the counterfeiting operation. It's not very broadly useful, but it's irreplaceable. Needed for the Hitman ending.
Finance: Do you care about money? Of course you do, and this skill makes your money go farther by saving it from the IRS' greedy maw. Also, majoring in business gives you a profitable racket to run. However, while this skill is the single best skill for earning spending money, it has very few uses besides that, and all of them are better covered by Persuasion. Still, money is highly versatile; while you pretty much have to be an idiot to not be able to pay Harcourt his last tribute, nearly every ending can be made possible by throwing enough money at it. This skill is almost completely useless if you've flipped, either to become an informant or agent (though you may as well take the business major if you want to join the FBI, because the other options are Tier 4), because the IRS can't get you. Useful for the Hollywood ending. Throwing money around is useful for the Good Life, Boss of Bosses, Senator, Suburbia, G-Man and (if Freddie is still alive) Hollywood endings, and for avoiding the Big Bang, Big House and No Escape endings.
Tier 3: These skills are situationally useful. They may find a use in your particular strategy, but their absence can all be covered up for by something better. Still, don't be afraid to take them if you like the idea.
Technology: Technology is Tier 2 in the early game, but you only need a small amount of it to do what you need to with it. It can get you an extra bookie, a little extra citizen rep, and save your bacon with the Sato attack. In addition, the Tech Support career leads to a useful racket where Technology is indeed a useful skill. However, Technology can be a pain to raise, and there's usually better skills to be training. Useful for no endings.
Automobiles: This skill can save your toosh in the early game when dealing with the Sato attack, and it also provides good opportunities for money (including a chop-shop racket, should you be a mechanic by trade), Respect and discount assassinations later; it's singlehandedly the reason why the Tier 4 skills are redundant. However, it's also a skill that's itself easy to make redundant. Useful for no endings.
Fighting: It has its uses, particularly in prison, but in the late game this skill has no real steam, and what uses it does have can be covered by Firearms. In addition, its usefulness in prison is compromised by the fact that fighting in prison builds heat, which you'd probably rather avoid if you want to get out early. As an associate, it can also make you more popular with Dana, but Dana's not really worth courting. Theoretically useful for the Boss of Bosses ending, but in practice this is a moot point.
Streetwise: This skill just barely avoids Tier 4 for two reasons. While nearly all of its uses can be covered by something else, it's easy to raise by doing next to nothing. More importantly, Freddie can flip to the Feds, and if he gets killed, you can't have him scriptwrite for you in the Hollywood ending. So if you're going to go for the lower-class background anyway, you'll probably end up with enough Streetwise by sheer luck to make it useful. Useful in the Hollywood ending.
Tier 4: Novelty skills. Anything they can do something else can do better, and the professions that lead to mastery of these skills are not particularly lucrative. Take them if you want to be a crooked lawyer or doctor.
Legal: Okay, running a law practice can make you popular with the Family, but there's no money in it; the car accident can make money, but being a mechanic is more useful for that than being a lawyer. Useful for no endings.
Medical: Useful for assassination (if for some reason you have no skill with Firearms on a non-pacifist route) and a bit of quick-stitching. It can save your kid's life if you have to get him back from the Smirnovs, but it's a suboptimal solution and a sign that you've neglected more important skills. Useful for no endings.
Just my random thoughts because I was bored.