I think you would be hard pressed to find a retail business in Canada that doesn't accept US currency. It is however illegal in Canada for retail stores to make transactions in other than Canadian currency, meaning you won't get US currency back in your change. It'll have to be Canadian.
Now the other issue, when I take US currency to my Canadian bank they charge me 3.5% to exchange it. With the two currencies running close to par for the last few years, don't expect more than a straight exchange.
Just for interest sake, I was in Las Vegas last year at a large casino paying for lunch with cash. I had some Canadian pennies in my change and the restaurant refused to accept them. No quid pro quo there.