Well, a 'good' thumb position can be different for various situations. I don't have my thumb behind the neck all the time, too. It depends what I want to fret and what position makes it most comfortable.
Another important aspect is the rotation of the wrist. An inward rotation of the wrist makes it harder to reach certain notes:


An outward rotation helps to keep the fingers relaxed:


On wide stretches the thumb may even move to the lower edge of the neck.
Front view:


Side view:


Of course some chords require both, a good position and some dexterity:


Ok, I admit that was cruel.

And one last comment. If your pinky doesn't have the strength to reach certain notes actively you can help yourself by fretting this note first before you position the other fingers, so you can bend that pinky with the weight of your hand.

Marcel