Tipping, is it in the bill?
When Dinning at Guy Savoy or ADPA do you need to add extra to the bill for a tip?
In France, gratuity/service is included in the price for food and in the final bill if the menu indicates "service compris". Having said that, it is customary and expected that you leave a little extra for good service in the form of bills and or change, but not more than 5%.
Generally (in most restaurants) you would leave a few Euros (coins not notes) if service was good, but nothing if it was bad. This could be as little as €1 or €2 if you are in a bar bar or cafe. It is a symbolic thank you, because service is in the price, and staff are on negotiated/regulated wages, thus you don't need to tip generously to subsidise wages.
In higher end restaurants like these you can leave a little more, maybe a low denomination note or two, say €20 in total - but 5% would be an absolute maximum.
assume it's in the bill until absolutely and completely proven otherwise.
leaving a bit of money is fine, as others have mentioned, but 5% is really, really high.
I think the real answer is that restaurant tipping in France is "all over the map."
I have met and/or know people who never leave any sort of tip, in any sort of French restaurant, for any reason, regardless of the caliber of service given; they seem to be somewhat proud of having this approach.
Other people will leave a few coins or whatever change would be due, rounding up on the money they left on the table.
Some will leave a fixed percentage up to 5 or even approaching 10% in a very good restaurant where they have received exceptional service.
As a general rule, in a cheap restaurant such as a pizzeria, where you have received indifferent service, leave either nothing or perhaps up to a euro, especially if you don't have correct change and don't want to wait for the server to return with the small amount that might be due you.
In a better restaurant I would consider several things before deciding on what to leave. These would include the caliber of service you received, and most importantly the likelihood that you will return. If you are kind-hearted soul then consider the first one more than the last. I always tip more in places that I expect to return to, than in places that I know from the start are a one-shot experience.
Most of the French residents never tip. Or, if they do, it's just a few coins. "Service Compris" is actually a regulation in France. I respectfully disagree with the above poster that "it is expected".