oh and this bit is kinda physics/maths related but hey you guys might find it useful.
actually it is extremely physics/maths related and all its doing is proving why its best to play with your thumb in the middle of the back of the neck!
if an object is in equilibrium (ie its not moving, like a guitar neck while you play) the forces on it are balanced. that means that to push down hard on the neck with your fingers, your thumb must also exert a force on the back of the neck as well. if the force on the back of the neck was really weak or wasnt even there, then the maximum force your fingers could apply would be much less.
THUMB ---> |neck| T
its hard to draw on here but that is a diagram of the forces your thumb would exert on the back of the neck if it was to one side (at point T). The overall force would be the diagonal line going from X up the neck (N) but the important force, the one that will oppose the force of the fingers will be smaller as the angle at N increases, which we can call x.
I'm not entirely sure off the top of my head what the force your thumb would exert on the neck, but if it was say, 10 Newtons, and the angle x was 20deg then the force parallel to the neck (the useless one) would be:
Fsinx = 10 sin20 = 3.4 Newtons
and the force perpendicular to the neck (the useful one) would be:
Fcosx = 10 cos20 = 9.4 Newtons, which is less than 10 Newtons, which means the force exerted by your thumb is gonna be less than if your thumb was in the middle. Therefore, your fingers would only be able to exert 9.4 N as well, which makes accidental muting more likely :(
By increasing the angle further, to 30deg, then the useless force would be 5N and the useful force would be 8.7N, which is even less, and the force from your fingers would also be only 8.7N as well. :(
Here endeth the lesson :P
please dont think im some kinda maths freak cos im not lol. I just thought this topic needed a bit of proof as to why its a bad thing. Don't worry if you dont understand any of it, just remember to keep your thumb in the middle of the neck as much as possible if your fingers have got loads of notes to fret at one time.