For us it just depends on the circumstances. For private parks it is rare for us to reserve if there are a few to choose from along the way, plus it is our least preferred method for overnight stays. We prefer state, COE, city/county, and other federal options first before resorting to private parks.
We will reserve ahead if we want a specific site at our favorite public campgrounds or when trying to secure a site on a busy holiday/summer weekend. We do favor a few private parks that is prudent to reserve in advance due to the concentration of RVers in the area at certain times of the year.
We also enjoy not having to be tied down to a rigid schedule now that the kids are not camping with us as much. Just an indication is next week we are out RVing for 7 days and only have reservations for 2 of those 7 days, we know the area and will just wing the rest according to our whims.