GREAT post from anatomist at TheWeighTrainer:
That wider grip stuff is nonsense. I think that bit of folklore is actually derived from things like the fact that your back looks wider when you're doing them, and that the exercise is harder that way. The reason the exercise is harder very wide is due to mechanical disadvantage and making some muscles less able to contribute - that is, it works less muscle, hence you can lift less weight. Is this supposed to be a good thing?
Different widths and grips do emphasize different muscle groups, a little bit, but I am skeptical as to whether you would see this difference in the body. It makes more difference if you want to apply the strength developed in the pullup to, say, rock climbing, where a pronated grip is more useful. The best grip is a comfortable one that you can keep using without injury. In general, I find the most comfortable grips are shoulder width supinated, and narrow parallel.