My experience is that the Canadian Rockies, Denali and July 4 are the biggest concerns.
However, I've also found that if you are flexible enough, its not an issue..
By flexible, I mostly mean the following:
1. Occasionaly, Be willing to accept a dry site rather than hookups
2. Occasionaly, Be willing to wait a day or two to get in the popular spots
3. Occasionaly, Be willing to boondock which is easier in Alaska than most any other place..
Edit...When I said "the Canadian Rockies" above, I really meant Banff to Jasper...not the Canadian Rockies farther north....