I believe the rational for waiting to look at your cards until your turn is so you can watch each of the previous players in turn as they make their decisions (so you don't miss anything while looking down); and so you don't give off any tells before your turn (i.e., you look at Aces and get antsy and prevent a early limp or whatever).

I have read somewhere (sorry, forgot where - maybe Jones?) not to wait at low limits, because you don't want to slow down the game, or give the other players any indication that you think about what you are doing. The first part makes sense to me; the second part - whatever - I'm not sure if it really matters, although I'm sure whomever it was that wrote it probably has a lot more experience than me.

I think I've also read that it is best to look at card in 7stud early, b/c it makes it a lot easier to count relevent cards from players who fold b/f you (although if you are proficient at counting the whole deck I guess this isn't an issue).

I think those are the pertinent arguments. In the end I guess it's personal style - a matter of how easily you give off/read tells; and how much time it takes you to make a decision. The stud argument is probably the strongest, but doesn't really apply to hold'em.