Different for different people and different guns.
Assuming you have a proper grip and the firearm is properly aligned with your forearm, the ideal would be to place your finger on the trigger blade in such a way as to be able to pull it straight back without any lateral pressure. This position will very much depend on the size of the shooter's strong hand, their finger length, and the trigger reach. Because of the way our finger joints are hinged, if you insist on always using the pad of your finger, you may be pushing the trigger more to the side than straight rearward, if you have big hands and/or the trigger reach is short. This is much more likely to pull the sights off-target than if you simply apply more finger to the trigger.
But sometimes the ideal position does not work. Heavy triggers will require more finger to be applied for leverage and this might result in a less-than optimal finger placement. Sometimes a compromise finger position is required for DA/SA pistols that have two trigger pulls of much different weight that start from very different positions.
