I'd plan on two kinds of visitors. Those stopping by to use facilities, see sites, etc, and those staying. Racks would be in different locations for the different types of users. As was mentioned those staying will want their bikes with them in their camp site. Most often they lock to a picnic table or tree, but a U rack might be nice there. Elsewhere, like outside office, showers, beach, and other facilities and attractions, U racks are also nice since panniers are not a problem with them.

Personally I usually find I can improvise a place to lock the bike in the campsite, so racks in the other locations may actually be more important.

Basically any rack where you lean the bike against the rack is fine as long as there is space for the width of the panniers. Take care to keep U racks (or other lean type racks) far enough apart for loaded bikes.