We traveled from California to Florida last year. I input the final point (Disney), and then added points in between. I used mapquest and an excel spreadsheet. I made reservations for disney, universal, and a few very popular places (New Orleans, beach towns, etc...). Other places we just went with the flow.
We tried doing going with the flow this summer while driving up to Washington, and a lot of the Oregon state parks were full (on a weeknight!!)...even in the middle of i5?! They do have lovely and cheap campsites, though.
My kids love long road trips....unlimited movies and video games. :)
We are now planning our trip to Yellowstone for next July. I made reservations and noticed the larger sites are already booked! Luckily, we have a smaller trailer.