The advice I've seen given to American photographers, especially after a blowup over pictures, is that if you photograph a crowd, you don't need a release (it would be unreasonable to expect you to track down everyone in the picture). But if you photograph a subject or small group, you're supposed to ask if you can, and if you're doing it for commercial purposes in particular, you should just have a model release form handy and ask them to sign it on the spot.