For people with experience using taxis in Paris: do taxis in Paris take credit cards, or do you need euros?
Some do; some don't. Ask before boarding.
Another possible wrinkle is that credit cards from the USA (even with a chip) don't always work. We encountered this on highway tolls -- some worked, some did not. We spoke to the disembodied voice on the microphone the first time this happened, and she asked "Are you American?" "Yes." "Oh -- we don't take American credit cards."
Be prepared with Euros in cash, just in case.