We are usually no reservation people however sometimes & some places reservations are wise to have. You can wing it for most CGs in Denali however Teklanika is always in big demand so it is wise to reserve early for that one. Fairbanks for the Summer Soltice is a big deal so reserve there if that event is in your plans. As anywhere July 4th is a big deal. Be wherever you are going to be early or make reservations. If you are a fisherman or even if you are not there is high demand when various seasons start so a little knowledge is useful for when & where to be or not to be.
In Canada watch out for July 1st, their Independence Day. Every Canadian who has a scrap of anything that resembles camping gear is out. The towns are abandoned.