You need the thumb behind the neck for stretches, in classical music it's required. You need the thumb over the neck for bending, watch Hendrix play. I don't think that it's a good idea to do it one way but rather use both.
Steve-0
Much like most of the people in this thread, I would agree that both styles are useful depending on the situation. I play mostly folk and finger-picking, so I don't play a lot of lead guitar. Because I have arthritis, my index finger starts to hurt if I play chords completely barred, so when I am playing non-open chords I used my thumb around the other side of the neck to get the lower notes (generally the low E string, and sometimes the A, they ae at the same fret for the chord). For example, with F and Bb chords, it hurts like Hell to hold down all the strings with my index finger. On top of that, if I were to use individual fingers to form the chord, I wouldn't have enough to cover all the strings... so I end up using my thumb quite a bit. When I play lead however, my thumb goes back to the back of the neck for speed and agility (and the fact that I'm not using it when I play lead, so there's no need to keep it wrapper so far around).
It really depends, I use both ways for both lead and rhythm. Having the thumb over for rhythm can be real usefull. For example when playing a Fmaj7 like this [1 3 3 2 1 0].
