Just a little clarification on this. You need to check the bill. If it says SERVICE CHARGE INCLUDED,
SERVICE COMPRIS in French language it means tips are included. If there is no mention of service charge then tips are not included. Unlike in America,in Europe waiting staff do not rely on tips to make up their wages. Tipping is optional and you can even refuse to pay the service charge if it has been added to your bill if you are not happy with the service you have received. If there is no service charge, you are not obligated to tip, it is very normal to just pay the exact cost of the meal. I have worked as a waiting staff so I always tip but its never as much as I do when I am in America. For example, if a bill comes to €17.50 I would round it up to €20.
This is a great article which explain is well
http://gofrance.about.com/od/travelplanning/a/tippinginfrance.htm