I had the chance to spend a day in Toronto with my camera a couple of weekends ago. I happened to hear of "Graffiti Alley," a back alley immediately south of Queen St. West between Spadina and Portland, so I went looking for it.
I wasn't disappointed and it was interesting to see some of the steps Toronto is taking to provide a "canvas" for graffiti artists in order to reduce the amount of ugly paint there is in the city.
Since the local business association seems to be ok with this, I guess it's a good thing. Otherwise, I think the rule should be quite simple: if you want to paint the outside of a building, start with the one you live in.