Often times we arrive early and don't recall ever having a problem. If the site was still occupied, we would just head off on our excursions and check-in later. I guess it's not a big deal since we have a truck camper. Arrival and departure is pretty simple.
During my many years in the hotel business, as a long as a room was ready, it didn't matter if a guest arrived before check-in. Our philosophy was they just saved us from having to do the work later.
Checkouts are usually tough, but only because I hate heading back to the reality of work.
The bigger issue we've encountered, are with those that think they own rights to select areas of the forest and try to bully their way into claiming a site when they're not even there. We've even seen large trees moved across access roads into prime campsites. After a visit with the Ranger, we learned the site was available for use. Unfortunately, we've seen these types of shenanigans on several occasions.