Been to Alaska 3 times. Reservations are generally not needed. Get where you plan to be by mid afternoon & you should be fine. Avoid the full service CGs where the tours flock to & make reservations for the July 4th holiday or get there before the locals do.
Always travel prepared for a few nights of dry camping. JMO but the provincial parks & state parks are the best anyway giving the northern feel. The commercial CGs are much like anywhere else.
For Denali make reservations, especially for the Tek CG, the best one to be in by far.