Yeah, you don't have to account for bullet drop. I don't know if it's there, but at least it's not notable at the ranges you usually fire from. But you do have to lead sometimes, especially against cav.
I generally try to get as much of the player(s) I'm aiming at as possible inside my sights reticle (or whatever you should call it). You will always hit somewhere within it, so the more stuff you get inside it, the more likely you are to hit something. You should always aim at clumps of enemies if possible.
That said, I don't know if you have a higher chance to hit in the centre of your sights or not, but I don't think so.
When I'm firing at horsemen from a distance, I always aim for both the horse and the rider, that way if I miss the rider I will likely kill the horse at least.