How many make park reservations vs find as you go.

I will go to Reserve America and snoop around at different times and see what campgrounds are getting filled up for a particular time and which ones are not. I use that as a guide whether or not to make a reservation or if it looks like there will be plenty of spots available.

I will also use Reserve America to give me an indication of where to stay away from. Full all summer? I need to go somewhere else.