I used to think most people single tapped with their index until the keyboard status overlay came to be. As for me, I started single tapping with my index, then a few months later switched to middle finger, then back to index and finally back to middle finger again (yeah...), at this point I can single tap with either finger just fine, even if my middle finger feels more accurate.
I still prefer to alternate most of the time though, it just feels more natural since I've been alternating for a while now.