This is an ongoing debate unfortunately; a debate for which there isn't necessarily a definitive answer. Through my own experience, and from the questions I have asked, it seems that it simply pertains to what you are playing and what is most comfortable.

A more formal approach, ala classical, may suggest you play with your thumb behind the neck all the time and keep it parellel with your fretting hand pointing inward. However, the rock gods we all know and love use their thumb all the time and you will see this technique across the board in other styles as well.

In my opinion as long as you keep your thumb pointed inward, whether that be behind the neck or wrapped around, you are good. Don't neglect how you are sitting when you practice, or your posture either. Those things are just as important to monitor.

I checked out some of the shred lessons on the site, and each of the teachers seem to apply different techniques. Some keep the thumb behind the neck, and others wrap it around. The tougher shred stuff will take more of a behind the neck approach, but the more bluesy licks and pentatonic licks are helpful with the thumb. It applies to what you are playing.

Hopefully this helps. :D