As John said, not too necessary to have reservations most of the time. During salmon runs on the Kenai, 4th of July in Seward, Denali NP campgrounds and if you have a specific place you want to be on a specific date. Seldom do we make reservations more than a week out, most of the time it is a day or so ahead of arrival. A few times over the years we have had to find a dry spot to camp but not too often.
There is a music festival that takes place in Dawson Town in the summer and reservations might be needed. When you get into the north country stop at the different visitors centers and pick up brochures of the different towns. They often will have the dates of festivals, etc that can fill the campgrounds. Ferry reservations tend to be needed on some of the runs but often a day or so ahead is plenty.