What am I supposed to do with my thumb?!

Hi all. This is my first post! Cool site.
I'm into the shred type playing, and I'm getting ok stamina wise and at sweeping arpeggios (perhaps not quite as cleanly as i'd like!)
But I have no idea of the best thumb position. i was watching the Michael Angelo video (speed kills), and you can hardly ever see his thumb over the fretboard. Mine tends to stick right up or even curl over the top of the fretboard (I know this is bad technique!).
Any hints anyone?
Cheers

There is no such thing as bad technique.
Everybody's hands and fingers are different so what works for other people might not work for you.
Hell, if I paid attention to half the guys who told me I was using the wrong techniques, I wouldn't be able to play half the stuff I play now.
And that includes where the thumb goes... move it to where it works best for you depending on the chords, the style of music, your finger length and size.

I agree with Hootyah. Some techniques may look fancy but don't sound too great while others can look really sloppy and sound times better. Do what works for you man.

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.