Margo - I agree with the flexible perspective, or being able to place your thumb on the neck to suit what you're playing. One thing I didn't see mentioned is the idea of paying attention to the amount of tension created between your thumb and your fingers as you're chording or playing notes. It's very possible to injure muscles by using too much tension, and tension prevents you from being able to move quickly or gracefully around on the fingerboard. Some of the tension is just there at the beginning because it's a new position for your hand to take and somewhat unnatural. However, if you notice pain in your wrist or the thumb joint and you've been playing for a while, check to see how hard you're pressing on the fingerboard and if the energy of the fingers is opposite the energy coming from the thumb (if that makes sense). One other thing I notice is that if I'm playing single notes on the high e and b strings, my thumb is usually more on that side of neck. If I'm fingerpicking, my thumb tends to be in the middle of the neck.