Quite ok, but the problem is, if you ask the permission before, the moment/photo is already ruined. Usually if you carry around a camera and see a good situation, you have to act swiftly and in a candid way, because when people see that you are photographing them they will throw the fake face which again, ruins the photo.

So, you can ask their permission after you've taken the shot. And if it's a very good photo and the person says no, well, you can still have that photo for yourself.

I don't think anyone will file a model form in public after you have taken their photos.