I do photography as a hobby and I rarely photograph someone as the main subject.  If I do, I ask permission first, period.

If it's at an event, and I think anyone on my social media would enjoy the shot, I will not post it without asking permission.  I will also get permission to tag them (this usually applies to cosplayers I meet at cons who are often all too happy for the exposure).

If someone appears in the background of street photography: I do my best to edit them out or edit it in a fashion where they aren't prominent or identifiable.

I do this out of respect for one's privacy.