We do a little of both. Typically if our destination is a national park, we'll make reservations well in advance, but don't make reservations for campgrounds along the way there or back. I usually plan an extra day to give us flexibility getting there. If we see something we like, we'll use the planned slack time as needed.
When trips are primarily for exploring an area, we don't make reservations at all.
When we started traveling by motorhome about 20 years ago, we felt more comfortable making reservations for every single night in advance. However, with experience and advances in technology (making finding a nearby campground much easier), we started enjoying less planning and more winging-it.