My reservations have always included the line

Warning Messages
INTL DRIVERS LICENSE MAY BE REQ. FOR NON RESIDENT RENTERS

I've had them insist on having it in Florence, I don't remember if they asked for it in Rome or not. In Florence, the person before me did not have it, they would not give him a car.

i believe this is a case of where it's much better to have it and not have them ask for it, than it is to not have it when they ask for it. To me it's well worth the fifteen bucks to get one, even if they don't want to see it.