On a regular rental there's no penalty for early return exactly, but it in certain circumstances (as in the above example) it can result in different rates if you no longer qualify for a weekly rate or weekend rate or whatever. Returning early causes the system to reprice at the proper rates if you don't qualify for the one you originally rented under.

In the general case of 6 days to 5, there should be no change in rate because 5 days is the minimum qualification for a weekly rate. BUT WAIT. There may be at least one caveat: If you pick up on Monday morning and return Friday night which is technically 5 rental days, I'm going to bet that might not qualify for the weekly rate at least in some cases if not most/all. So do be aware that there could be some pick up day restrictions to the weekly rate as well. If your 5 days includes a whole weekend you should be all set for returning a day early.