I would make reservations as soon as the window opens, if nothing else so you can get good sites. Fort Stevens and Beverly Beach are two busy campgrounds.
Do be aware that time of year that Honeyman H Loop us used by the off road community. Staying closer to the front will be quieter, but since there are other private campgrounds along the dunes it can be a busy time of year, particularly on weekends.
Harris Beach: Starting October 1, 2016 A and B loops will be first come, first served until the whole camp becomes reservable May 25, 2017 for the summer season.
All D loop sites will be closed until March 16, 2017
Most C loop sites, All Yurts, Meeting Hall will be reservable through the Fall/Winter season.
If you are looking for a private campground right on the beach we really enjoy Sea Perch RV resort. Not as inexpensive as Oregon State campgrounds but the view is amazing.