My opinion page addressing inquiries concerning clawhammer technique.
I believe that you want to be as relaxed as possible when you play (I always tell my students that "tension is the enemy!"). So if "cocking" the thumb creates tension, then don't play that way! If "cocking" the thumb feels relaxed and comfortable, go ahead and try it.
I play with a "straight" thumb and have no problem with the volume of my drop thumbed notes. In fact, my "straight" thumb position helps me avoid overpowering the melody notes with my thumb, a problem with some frailers.

Thanks... Mike