A lot of it has to do with the airport. At LHR in T4 on the 747 I've always boarded thru the first door. In T3, most of the gates have 2 jetways, same with FRA. At DUB I've always been on an A321, and they have the jetway going to the first door, and a staircase going the third door, for people sitting at the back. At YVR, they always board thru the 2nd door if possible.