One house I delivered to kept getting parcels and when I rang the bell AND knocked on the door and there was no answer I always left a card. He then kept phoning the PO up complaining that he couldn't hear the bell or door as he was gardening. In the end he decided that he would get a 2nd doorbell fixed up that was for the garden. So now, whenever he gets a parcel or signed for I first knock on the door, then simultainiesly press both doorbells, which seems to make him grin like a chesire cat. When I told a friend about this behaviour he suggested to me that maybe he had fixed the doorbells to some kind of electrodes which were then attached to his nether regions, which gave him some kind of excitement- hence the big grin on his face. Now, whenever he has a parcel for him and he comes to door grinning away, I have to try very hard not to guffaw in his face.
Seriously though, the truth is whichever you do, knock, ring doorbell, or both you can be assured that it will be wrong for someone along the line so just do whatever makes you happiest. But: you really should know each households preferred method of alerting them to your presence because as you well know they are your ONLY customer as all postmen only deliver to one house each.