I lean towards the point of view of "most photographers" in the original parent comment, in that you need to always be respectful of the subject but there are c

I lean towards the point of view of "most photographers" in the original parent comment, in that you need to always be respectful of the subject but there are cases you can't get permission beforehand, and so yes I think in general you should ask for permission to take a photo of someone, but sometimes it's either not possible, for example if someone is high on Tranq, or will create the wrong conditions for the photo you want to take, like if you wanted a candid shot of someone on the street. My view is that in the former, you should have a legitimate reason to take the photo (like you're doing documentary work as mentioned), and in the latter you should ask for permission and delete if rejected, but it's fine to do it after the fact.