The optimal position for the thumb is in the middle of the neck in aprox. the center of the fingers:

n n n n <-- Fingers
------- <-- Fretboard
...U <-- Thumb

(hope that lil illustration worked)

so much for the theory..

i personally have my thumb there:

n n n n
---------
U

when i solo in hte higher frets i have it too reaching over the fretoard like you said, but when i´m on the low strings i got it in the center of the neck. works kinda automatical for me. *shrugs*