With a husband who works in the restaurant business, I can assure you that the "extra" tips are appreciated (and they aren't making all THAT much money, although yes they are better cared for than in the States. And of course they themselves also contribute to their insurance, transport pass, retirement, etc etc., it's not that it's all handed to them on a silver platter. Plus they work insane hours in most places -- the 35-hour workweek does NOT apply in restaurants, and while they may be supposed to get a break in the afternoon between lunch and dinner services, the one who comes home to me works from about 11:30 in the morning until 1:30 in the morning five days a week with office work in between his shifts on the floor. That's a lot of hard work, and with rent being so high here in Paris at least, even being a two-income, no child family, we don't really have very much money left over for very many "fun" things. I'm not complaining, my husband loves his job, and we chose this life -- but just trying to paint a realistic picture).
On a 78-euro bill, I probably normally would leave 80 euros -- unless I was at a place where I'm a regular, or I was really happy with the service, where I probably would leave either 5 euros on top of the 78, or maybe even 85. But that's pretty exceptional.
Rounding up to 80, you look courteous and appreciative, while not looking like an American sucker who added 10% or 20%!
: ) And yes, to emphasize again, it's not required to leave ANYthing. But it is nice if you're able to do so.