Our experience:
If you're looking for a overnighter, reservations are usually not needed. Most campgrounds have one or two sites they "save" for those people. Along the coast in California and Oregon are exceptions, especially during the summer.
If you plan on staying longer than one night, you most likely will need reservations and most likely you'll not be successful finding anything along the coast of California on a Friday/Saturday night.
A lot depends on the size of your rig too. The larger your rig, the fewer spaces available to you.
We reserve 99% of the time......good luck...Dennis