I would rather they ask if I want my change than to assume I am tipping the whole amount. Most of the time I will tell them to keep the change. But then again, if my bill comes to $20 and I only have a $50, of course I want my change. I do try to have smaller bills on hand for the tip when we dine out since most of the time I will put the meal on my credit card (racking up cash-back for vacation).