When I first arrived in Toronto for a vacation back in '02, the exchange place refused to change out my US coinage, saying "People here will just take your American coins. No issue there."
I pointed out that that doesn't benefit me at all, because if I gave someone a dollar in American quarters, I'd be undercutting myself by 60 Canadian cents (at the time), so I just left all my American change in my hotel room for the remainder of my trip. Of course, on the same note, since banks down here don't exchange coins, I still have a bag with $2.50 of Canadian coinage in my sock drawer as a "souvenir" of my trip.