Funny, you should bring this up, Nedland. This is the first question my DH always asks, when I book our Disney trips. He hates that extra day charge.
He loves, however, when I can arrange flights for our 7 nights, but at times that only require a weekly rental without the extra day.
For instance, if your return time on the car is before the time you picked it up.
Or, as in the case of my most recently booked trip, we leave home on a Saturday night, but don't arrive at MCO until midnight. Therefore, my rental starts on Sunday, and we will be returning it late on a Saturday.