Let it be said that, except in overcrowded cafés and restaurants, French waiters love tourists, because they generally go for dinner early (before us) and they quite often over-tip!
I mean, there's nothing wrong in leaving a big tip to your waiter if he was nice, but you don't have to leave a certain percentage of your bill because your waiter supposedly already gets 15% commission on everything you order.
Except in a Michelin-star restaurant, average tipping in restaurants is generally between 5 and 10€ for two to five people. And if the waiter was no good, you leave nothing!
And, by the way, it is not true that French waiters are rude to Americans: French waiters are rude to everyone! Actually that's not true either: there are some very charming French waiters, it's just saying that they can be rude and, if they are, they will be rude to everyone, me included. You see, that's the kind of problem you get for paying the guy in advance for a job he hasn't done yet and for which he'll get paid no matter what.
On the other hand, in America, most of their salary depends on tipping and that makes a big difference. But the big drawback is that sometimes the waiters in the US are too freaking nice. I have myself quite often said to my guests in American restaurants: "if that guy comes again before five minutes to ask if everything is OK, I will punch him in the nose!"
I'm so glad you asked!
As far as I and the other guides I recommend are concerned,
a tip is always hoped for but never expected.
The way I see it: if you leave a 10 or a 20 € bill to a waiter in a restaurant just because over a period of one and a half hour he did his job by bringing diligently to your table the dishes that you ordered as they came out of the kitchen and for which you already paid him for, what's wrong in showing your appreciation to a guide who has done his best at entertaining you by talking to you almost non-stop for the whole day?
And let it be known that, besides flowers, chocolates, handshakes and hugs, banknotes are still the most convenient and appreciated way of showing your gratitude for a service well performed!
Now, on a full-day tour or several-days-trips, if you buy your guide a very nice lunch (or dinner) and an ice-cream (from Berthillon of course), that can also do the trick!
And, at the end of the day, if you just shake his/her hand and put on your best smile showing your utmost satisfaction, you surely won't be remembered as "the best client ever" but I should say that it's good potential to make him/her feel satisfied for that day spent with you...