yeah, i tend to do a bit of both. Started by just taking candids but then started asking people as well. The trouble with asking people is that a) they might pose to much and b) they might say no - then you really can't take a shot of them.
I had a few instances on sunday where i saw some great characters and ASKED them if i could take a photo but they refused. I'm wondering if i should have just taken a quick snap any way, without asking.
I always use AV mode for street photos. And set my iso depending on how light it is outside really. Never use flash