I've never had a server get ticked off when I didn't leave an American-style tip. But yes, many times they do expect it, because so many Americans don't understand the tipping system in France, and tip as they would in the US. That's why you don't ask if the tip is included, as they will say no. And technically, that is true, as service and tip are two different things in France. The tip is that dollar or two you'll leave in cash. The service is included in the menu price.