GMF -
I think it's dependent on return mode and company. In fact at least one res agent encouraged me to do it this way and it worked.
I've used the UA50% paper successfully on National and returned after only 1 day when the deal was for a 2 day keep. This was at ORD. No grief.
I also believe that I used same deal with Budget at DEN.
I'd say do it.
Greg