In both cases, they did not ask for it. I generally have a habit of just shoving everything to the employee at the car hire desk and letting them figure it all out (passeport, carte bancaire, driving permit, and IDP if applicable). I think in some tour books, they say a driving licence from your home country at least for the EU is sufficient. Since I already had the IDP from prior years, I just bring it with me anywhere. They certainly photographed/documented it.

So I would say : if you don't have a international permit, then your French one would be fine and don't fuss about getting one if it's a hassle at the préfecture. If you have one or can obtain one easily, why not.

And off-topic, but despite what the internet and tour guides say, I would still get an adaptor for the Italian type L power socket. Major hotels will have a special outlet or two that fits our standard two-prong Euro plug, but the Euro plug will not fit the more widespread type L as the spacing is different. I'm glad I brought mine.