I was a street magician when I was at college in Boston. I hung out with several other guys. Buskers can be a skittish lot when it comes to photos.
Most guys don't care much about the casual snap shot. Your average tourist shoots photos of everything they see. As a street performer, you have to expect that. However, every guy has his own opinions on having his picture taken.
There was one guy who I only knew as "Sam" who would only let you take photos if you gave money. However, if you paid him, he would pose for you. Another guy named Peter Sosna didn't care one way or another so long as you didn't interrupt the show. There was a guy named Sonny Holland who was an old time card man who could do the best split fans and back-and-front palming I have ever seen. Sonny was a recovering alcoholic, the kind who is always "in recovery" and he could get temperamental. If you just did a run-and-gun shot he would scowl at you. If you took several pictures he would stop what he was doing and just stand there until you were done. He did not like video cameras at all and he turned into a real bastard. He sometimes hollered at people with video cameras.
My advice agrees with the above. Tip the guy and you probably can't go wrong. As a busker, I'd say tip a little above average, at least. Watch and see what others are putting in the hat. If they are tipping with change, put in a bill. If they are putting in singles, tip with a fiver. If you cough up a ten-spot, the guy will probably let you hang around and shoot for a while.
As with any performer, most guys would probably jump at the chance for some good publicity photos. Exchange business cards with him and offer him free copies in return for permission to publish.
That one guy, Peter Sosna, would probably love to have some really good photos for his website: http://www.petersosna.com/