I know of several instances of counterfeit USD being given to girls (it is far easier to counterfeit USD than CND), so some might refuse it, although I'm certain they would be grateful for the exchange rate as a tip.
Most girls probably prefer CAN$. Why? Exchanging money cost money, takes time and, worst, it leaves a trace. Since most girls probably prefer to leave no traces...
Hey! Why not! There used to be a motorcycle garage I knew who agreed to be paid in Canadian Tire money! The owner's mentality was that, after all, he did shop at Canadian Tire too!
I've always paid in USD...if the USD exchange rate is higher I give dollar for dollar...if the USD drops below the CAD (not to often) I give the USD plus the differance and throw in a little extra for the inconvenience.
I've only seen independents and have always asked first.