Anders Jacobsen has some trouble with the weird French tipping methods:
First of all, I really like a lot of things about France and the French; I really do. But if you're just a visitor, and especially if you know no French, be aware that the French restauranteurs/taxi-drivers/shop-owners/etc will all do their best to screw you. Sorry, but it's just a fact. At least here in the south.
Thanks to soften the generalization by limiting that to the south, because this is certainly not the case everywhere else. I lived and worked two years in that area (Nice - Cannes) and yes, it's how "commerce" is done there. Actually since anybody who's not from there since five generations is a tourist, they will try it on French too so it's not because you don't speak the language (but it makes things easier for them).
Tipping in France is such a complex business, almost taboo, without any written rules, that it is a mystery for the French in the first place. Unlike many other countries where tipping is part of some common rules, everybody in France has their own personal rules. There is only one occasion I know of, which you don't see much anyway, with small movie theaters where the ushers earn nothing but tips. Even public toilets, except a minority, have moved away from the "dame pipi" and her small change plate business.
Tipping is sometimes a marketing art, like in some US restaurants that print out a list of 10, 15, 18%... on the bill so you don't have to calculate it yourself, and even feel sorry if you don't leave that much. I have seen many times, in NYC restaurants, a 15 or 18% tip already included in the bill, following exactly the French model!
But at least we do have a common sense rule that I think is the same everywhere: there is no tip for bad service.