The answer varies depending on where you want to camp and the type of camping you like to do. I know of non-reservable NFS CG's where we can go on a Sunday through Wednesday morning and get spots. But if you're a family on your annual two week vacation to see some of the National Parks, you should plan on getting reservations early. Otherwise, you won't get into places like Arches or the Yosemite Valley. Often a family wants to camp in the park rather than at a RV park outside the National Park and having to commute in every day.