The Power of 'Bonjour'
The most common mistake visitors make in Paris has nothing to do with tipping or geography; it's a failure of basic interaction etiquette.
The Rule
Whenever you enter a boutique, a bakery, a cafe, or approach a counter, you must make eye contact and say "Bonjour" (or "Bonsoir" after 6 PM). It acknowledges the humanity of the worker and establishes equal footing.
If you walk up to a waiter and say "A coffee, please," you are treating them like a vending machine. They will likely respond with cold, slow service. Say "Bonjour, un café s'il vous plaît," and the interaction changes entirely.