Some good points made so far. Reservations may be wise on weekends, especially near cities (a lot of folks like to "escape" for the weekend!), and I would advise them in/near national parks and other tourist attractions, and on holidays. Having a bigger rig can make a difference too, as jamanatee pointed out. The bigger, the more you'd need to make sure a site is available that you'll fit into - and the way to make sure is make a reservation. Traveling during peak summer camping season can also affect you; CGs may be 1/2 empty now, but it's not Memorial Day yet.