How important are resv at campgrounds on Alaska Highway

We did the Alaska Highway last year leaving Northern Indiana May 19th, we returned August 4th, no reservations that I remember at all. But we were told we were running: "about 2-3 weeks ahead of the caravans". On our return we traveled the Trans Canada Highway from Prince Rupert BC to Kenora Ontario, then re-entered at International Falls MN, again without any reservations.