Dave Wirth
To me, there is no hard and fast rule as to where all guitarists should put their thumbs when fretting a chord. As far as I can tell, I have seen enough people play guitar in enough different ways to realize that any technique that doesn't hurt the guitarist's hands is good. The trick is figuring out what works best. The fun part of my job is the tweaking I get to do. Since every-one's hands are different, I get to approach each lesson in a fresh way. It keeps it fun.
The oft-repeated advice, "Keep your thumb on the back of the neck at all times," is good only for so many people, i.e. ones with the perfect length of thumb. Thumb lengths differs from person to person. The other issue is that the widths of guitar necks also differ. Therefore, this advice is not necessarily good for everyone. When the thumb stays static, the wrist wants to make up for any differences. When I say "carpal tunnel is not fun, and neither is tendinitis," please believe me. After about eighteen years of playing guitar, I know that the less bent my wrist is, the less chance of any injury. Sitting out is no fun, and I've done that twice.
The way out is to look closely at your wrist. Is it bent towards you when you play? Away from you? There's far more chance for an injury, in my opinion, if either is the case. If this is so, then take an even closer look at your thumb. If it stays static, then try lifting it up in-between chords and see if it feels any better. Doing this tends to release any locked-and-stressed-out sorts of technique in the wrist, and might enable you to play longer. Bonus, and bonus.
