How many make park reservations vs find as you go.

We are going to Manitoba and Saskatchewan in early August through early September staying at a few national parks and provincial parks. We made reservations the first day we could. Checked back three days later and there were only three shoes available that we could fit in. We tried to make reservations at a national park in Michigan on the first day but could not get in. So our theory id's that if you want a specific destination reservations are a good a idea.