When is it OK to Take Photos of Strangers?

After posting my “Mexican Bus Stop” photo yesterday, there were quite a few comments and questions about taking photos of people and how those photos can be used. So since it's poll day, I'd like to gather the group opinion on taking photos of people you don't know. When is it OK to take a photo of somebody? Should you have prior permission? Should you shoot first and ask after the fact? Should you just snap away and not worry about it? Should you leave people out of it and take photos of something else? Or is it really dependent on the situation?

I realize that the topic is much deeper than can be covered in a five-answer poll, but try to vote for the one that best matches your opinion. If you have more to say, I'd definitely like to hear it in the comments.

When is it OK to Take Photos of Strangers?

Be sure to check out last week's poll on “Which Camera Brand Are You Loyal To?” I was right in predetermining that Canon and Nikon would win, but I didn't think it would be such a landslide. Canon came in with over 50% of the total votes, and Nikon with over 25% of total votes. Looks like Sony has some catching up to do…