Tipping

Do not tip drivers.

If you are unsure about tipping or not tipping, not tipping will always be correct.

Personally I would tip in the range of 5 to10%. Perhaps 60E total. There is an element on this site constantly promotes no tipping. Have no idea of right or wrong but if I received good service from the driver I would give him 5e.

Just round up that is all; don't make it awkward. Carry small bills so they don't have to make change.
I don't understand what is involved when you say : "if I get good service from the driver," that merits a tip.

I think there is a misunderstanding of the term "good service". Good service consists of the driver doing what he is paid to do, i.e., putting your luggage into the trunk of the cab, transporting you safely to your destination, and unloading your bags from the trunk. That service is paid for by the set fare. It requires nothing more than a "Thank You", so do not tip.

"Exceptional service" is what may call for a tip, i,e., stopping at an ATM or pharmacy for you on the way, or carrying your luggage up to a fourth floor walkup.

How would one describe "bad service" and can you deduct from the fare you agreed to pay when you don't get good service?

It is ugly American who impose the tipping culture of the US where workers are under paid and often have no health insurance or benefits, on to France where this is not the case. A couple of Euros if the person handled the luggage and was pleasant is fine; more than that and you just begin to build expectations that US tourists are rubes to be exploited.

I have given larger tips twice in recent years. On one occasion the cabbie turned and went back a few blocks when I realized I had not left the apartment key; he could have put the rest of the trip on the meter or otherwise charged up but didn't. On the other, my arm was in a cast and he came into the apartment lobby and carried luggage and then at the airport got me a cart and loaded it. That is the kind of extra service that merits extra reward. For an ordinary trip nothing is fine and one or two Euros generous.

Here is what Rick says about Taxi Tipping:

For taxis, just round up to the next euro on the fare (to pay a €13 fare, give €14); for a long ride, to the nearest 10 (for a €76 fare, give €80). If the cabbie hauls your bags and zips you to the airport to help you catch your flight, you might want to toss in a little more. But if you feel like you're being driven in circles or otherwise ripped off, skip the tip.

The same would apply to a private car service, from my own experience.