Thumb positioning naturally has your thumb reaching over the neck. This is the standard for blues, rock, jazz and country style of playing. That's because those styles don't call you to overly obsess about your technique. Shred on the other hand, is an extremely technical thing, as well as classical. The reason you won't see many shredders put their thumb over is because it increases your stretching ability. You can stretch further away from your "home" position if you have the thumb back there, rather than the thumb over the neck. Being the jazz player that I am, I never really think about this, plus I like to use my thumb as a fretting finger for bass notes in chords.

It comes down to you, but if you want to be able to stretch very far, maybe practice keeping your thumb at the back of the neck. Of course, this may seem very awkward. If you keep trying at something new, though, it won't be awkward anymore.