If the HB sites are full, the ranger may turn you away, at least I've seem that happen quite a few times in Socal.
This happened a couple of time to us last summer. Many of the Socal campgrounds have closed or tightly regulated their hiker/biker sites do to problems with transients. We were fortunate to get a regular spot at one of the campgrounds when the ranger told us the hiker/biker site was full. Anywhere north of San Francisco into B.C., campgrounds are not a problem.