I'm lucky to live in a country where almost every man, woman, child and dog has a mobile (cell) phone capable of shooting pretty good pictures...and boy(!) do the Japanese love to shoot pictures.
So no, I don't ask people if I can take their picture because 99% of the time, it's the spontaneous shot that truly captures the joy/laughter/sadness/whatever in someone's face.
Asking permission pretty much negates the point of taking the shot in the first place.
If I'm using a short FL, and the subject spots me, I often show them the shot and ALWAYS compliment them on their looks. So far, nobody has asked me to delete a shot outright. Some people are shocked at how much emotion I've captured, but after a chat, they see the value of recording said emotion.
And I truly believe we need to "teach" the public as often as possible not to fear having their picture taken by a street photographer, otherwise, what a dull time in history this will appear to have been to future generations...
Regards,
Simon