We are heading out west on Easter Sunday (April 17), a prettty long 6 week trip. I have reservations for 5 of the 6 weeks. At the beginning, we will wing it for about 4 nights, then all reservations from April 21- May 23rd. When you head towards popular areas, I believe you need reservations for peace of mind. Of course, you can be derailed by the unexpcted but that's the chance we take.
It is sad in a way that reservations are a must. We accepted it and feel if we really loved a new area, and wish we had more time to explore it, it gives us a reason to go back in the future. Good luck.