I get exactly where Tamara's coming from. For single action shooting, the pad of the finger works very well. For double action, one needs to place one's finger deeper on the trigger in order to get enough leverage to pull the trigger without torquing the gun.
95% of my shooting is with double action revolvers, shooting double action. When I shoot that way I center the crease of my first joint on my trigger finger on the gun's trigger. That gives me just the right amount of leverage. Shallower, and I tend to pull the gun to the right when I shoot. Much deeper, and I push the gun to the left.
I have a tendency to leave my trigger finger in the same position when I shoot semi autos in single action, as with my 1911 or my CZ 75B SA. It's not necessarily the preferred way to do it but with so much double action shooting I'm just more comfortable that way.
My wife used to have a Smith & Wesson 41, with an ultra sensitive trigger. She could make that gun do tricks for her, shooting tiny groups at 25 yards. It was an unmitigated disaster for me. After many years of shooting double action revolvers I never could develop the right touch to pull that gun's ultra-sensitive trigger. I was totally uncomfortable resting just the pad of my trigger finger on the trigger, so I reverted to shooting it with my finger deep into the trigger, in the manner that I shoot da revolvers. That also was a disaster, with numerous premature discharges and shots all over the target.
So, I'd say that where one puts one's finger very much depends on the gun that one is shooting. The basic rule, however is: the harder the trigger pull, the more deeply you need to put your finger on the trigger in order to be able to pull straight back without torquing the gun.