Personally, I am sensitive to this issue. I have posted photos of folks that are posing or working someplace where picture taking is the norm. However, I try to crop out or blur the recognizable features of by standers. If I have a particularly "good" photo of someone, I usually offer a print and then ask if I may use it for non-commercial purposes. My rule on this is, do unto others as I would have them do unto me.
I believe, asking first is always a good policy. If you can't ask, then I would not post a recognizable photo of anyone, no matter how good it is. I know this will sound far out; but, consider this. You see a lovely person and take a nice shot of them and post it. Unfortunately, this lovely person has been the object of harassment by another and has moved to escape the problem (not an uncommon problem these days unfortunately). So the harasser sees the posted photo and subsequently finds and harms the person that you posted the photo of! Far fetched? Maybe. But how do you feel about this, no matter how small the probability?
Anyway my 2 1/2 cents worth.
chuck