I've found that making my driving days in the middle of the week - I can almost always get a great spot in the destination campground.
Since we love to camp at COE, state parks and similar campgrounds, we usually are staying for a week after we arrive. We always reserve a 7 to 14 day stop for the major summer holidays - Memorial Day, July 4th, Labor Day.
We've also found that the relatively few good campgrounds in warmer areas are hard to get into for Thanksgiving week and the week after Christmas Day.
When traveling across country - we have a plan mapped out - and will reserve one or three days in advance depending upon our feel for how busy the area is.
We try to get to the destination before 3 pm each driving day.
The real decision on reservations is the time available. When on a tight schedule to get somewhere by a certain data - reservations are good. When you have time to stop and explore, or to change your mind and go somewhere else - reservations are bad.