We've always planned our trips with a full itinerary and reservations, and then make changes as we go (if the changes we want to make are available). Sometimes it means having to eat the cost of a deposit or two, but still worth it to us for the peace-of-mind.
However, one time we decided to make a route change in the middle of a driving day, and we spent 4 hours at a highway rest stop, calling approx. 25 different campgrounds before we found one with availability for our length rig for the next 2 nights. We ended up about 30 miles away from where we wanted to be. It's experiences like that one that compel me to make reservations in advance.