We did the same on our recent trip. There is a slight discount for renting all at one shot. I can't remember exactly how much, I want to say 10 or 15%, but hey, every little bit helps the magic go on! Although the counter help at the rental station was very pleasant and accomodating, they did not volunteer the info that you could pre pay for multiple days and I never saw it posted anywhere. It's one of those things you just have to "know".
Anyway, have a great trip!