Originally Posted by: schmangeIn my experience, it's different for everybody. Different sized necks, hands, fingers, playing styles... like I've said before, there's no set rules.
A lot of beginner teachers will suggest keeping the thumb placed behind the neck of the guitar but it's only meant as a suggestion to make it easier for them to press down the strings. Once you get more experience, you start to realize that your thumb is going to move around all over the place depending on what you're playing.
Basically, whatever works and feels most comfortable is what you should use.