I would do it before you leave, and continue to shop rates after you book, just as you would here.
The U.S. companies are fine in France and surprisingly, Hertz often has the best rate. Europcar has a reputation of being a stickler on damage.
Speaking of damage, consider your coverage carefully as there is a far greater chance of getting a ding there than there is in the US. The reasons for this are:
-Smaller parking spaces.
-Less well defined parking spaces.
-Spaces with strange markers, such as rocks.
-French drivers, some of whom are unlikely to leave a note if they ding you.
-Your likely lack of familiarity with roads, etc. there, plus possible jet lag.
I sometimes use the Amex $19.95 coverage for the entire rental as it is primary and I have heard Amex just takes over and deals with the rental company. Another option is a credit card that has primary coverage for damage to a car you rent-I think Chase Sapphire Preferred has primary at present.
I always call the card company before I go to check on the T & Cs needed to get coverage in place.