I don't take pictures of people without asking first. But there are exceptions:
1. If they are not a major feature then I don't ask (in other words, they are not the subject but are somehow in the frame--such as shooting the Lincoln Memorial). I can't ask a hundred people to get out of my viewfinder.
2. I shoot celebrities from time to time in public places. If they are near me I ask politely (never using a first name. That's rude in itself, which is what most fans do. I use their title and last names to ask). They usually say yes. If they say no then I lower the camera and leave.
I will shoot them from a distance as they engage in an activity without asking first. But it's important to me that I NOT engage in paparazzi-style photography. I have no wish to embarrass celebrities. So if they are frowning, picking nose, etc., I'll delete the photo. I'm also not going to get in anyone's face or block someone's path to get a photo.
3. If someone puts him or herself on display, then I don't bother to ask. A street performer, for example, shouldn't complain if I try to take a picture. I'll support them with a donation, but if they say not to take a photo I'll respect them with a comment, such as, "That's fine, but you are putting yourself on display in public." Then I'll turn my back and walk away.
Any professional sporting event that allows it is okay, too, along with musical performances, etc. I always respect rules (if a sign says no photography), but if someone is performing then they probably enjoy the extra attention and possible promotion.
Standard Street photography is something I don't really like to do unless it's an event and there's a strong expectation of street photography. I don't want to interfere with the privacy of another person, regardless of whether it would make a great shot.
In the rare situations in which I do shoot without asking, I follow up by showing them my picture and asking if it's okay. I say, "I thought this would make a great photo. Is it okay if I keep it? I'll delete it if you say so." They usually appreciate it and then ask for a copy.
It really comes down to the golden rule. Don't do anything you wouldn't want done to you.
Here are some examples of when I find it perfectly reasonable to shoot without asking.
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Here are examples in which I DID ask (because I was up close).
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