I typically make reservations for destination resorts or campgrounds by large cities eg: Salt Lake City, Portland, etc. Often along the way we will use the cell phone to make reservations for a few days ahead or even calling at noon to see if a spot is available for the evening. Much depends on your schedule. If you have two week of vacation and must be home by a certain date, then advanced reservations all along the way make the trip trouble free and predictable. If your flexible and need to be back sometime next month, then you can afford to travel stress free if you find your route changes. 
Happy Trails.