I always position my thumb to 'fit' with whatever I'm playing; never to support my arm.

Sometimes it is in the 'correct' position, or wrapped around and muting the 6th string, or even fretting it if the chord is easier that way (such as bouncing Barre chords to mute them).

Classical guitar is a study in its own right, so I think you have to position both hands and guitar correctly or you'll never learn to play to a good standard.