IMHO, that question has to be answered based on the individual's unique facial geometry. Not everyone's bone structure, i.e. cheek/jaw line is the same. If the thumb is down does it help/hinder settling into your primary anchor? What affect does it have on eye/shaft alignment? I can shoot with the thumb "up", but relaxed and flat against the base of the index finger (think pics you've seen of Fred Bear) and I can't tell that it causes any perceptible increase in hand tension. At one time I shot with it sticking almost straight up as a secondary anchor against my temple..this did cause tension. Having said all that, I now shoot with it down, but relaxed. My second anchor is now the thumb's base in behind my jawbone. this feels natural and for me is repeatable. But to make it work I had to slightly rotate the hand into my face (less parallel to the string). I have a bit of angle to the string fingers, something coaches mostly discourage, but it works for me. Find what works for you.