I usually make reservations but have had times where I didn't. One thing I have found is that if you don't make reservations you run the risk of not getting a site that meets your needs. With that said the camp site 20, 30, 50 miles down the road might have exactly what you need but that just adds more stress to the situation not knowing if the next place will work. You can spend an inordinate amount of time find a spot that meets your needs. If your needs are minimal it gets easier. Fortunately for us we always carry an attitude that we'll take whatever comes and adjust accordingly, less stress that way.
In high traffic areas during peak season I believe it's always better to have reservations than not.