My thumb is straight up, like a hitchhiker
relaxed and just infront of my ear.
I don't see alot of people shooting that way, but it works for me. My hand is more relaxed that way, and I feel I get a better release.
With my thumb tucked in I am more prone to torque the string. Also with the thumb out of the way I can get tighter to my face and the arrow is directly under my eye.