I generally make reservations, though often it's not very long in advance. I don't like the idea of arriving in the evening and not having a place available to camp in. Sometimes I could have done just fine without a reservation, and other times it is very good that I have had one. Whatever the case, I don't at all like the idea of arriving and finding no room at the inn, so to speak.
A fair few of the public campgrounds in this area have pretty small or otherwise constrained sites, and there are a good many in some parks that I would not fit in comfortably if at all (with a 32' class C motorhome).
Needless to say, the necessity of having reservations depends a great deal on where you are going, on what season it is, on what time of day and day of the week you'll be arriving, and on how picky you are about what sites you can use. There are advantages and disadvantages either way.
