The reason that most writings suggest the use of part of the pad of the finger
tip, instead of the crease or fold of the joint, is to do with biomechanics...
The goal is a staight back action on the trigger, thus avoiding a twisting or
torque force... it seems to be more easily accomplished with the pad...
BUT, like anything else, we're all built a little differently, and with such a
variation in hand and digit size, you've got to find what works best for you!