I think some of that depends on the campground location. If it's a campground that gets folks driving by looking for a place to stay the night, then booking an extra night and letting the host know you've vacated early means a drive up might get a site that otherwise wouldn't be available.
However, if the campground doesn't see that "drive by" visitation, then the site will remain empty when it could have been reserved and used that night.
In contrast, with the latter type, if a reservation hasn't been made directly after yours, then it usually isn't a problem if you ask the host if it's OK to leave late.
And what day you leave, and the season, will have a big bearing either way. If you stay during the week and want to leave late on a Thursday or Friday, you are likely out of luck. If you are leaving on a Sunday or Monday, then you'll have far better odds. And off season will give you more versatility than summer.
If none of that works, then you want to boondock so you can leave whenever you want.