I, too, find reservations needed only for holiday weekends, and then mostly at popular destinations. Places like Yellowstone, Grand Canyon South, Yosemite, require reservations always, but most places have open sites most of the time.
If you plan to arrive early in the day, say mid-afternoon, there's always time to look around for an alternate campground just in case the one you went to first was full. And frankly, arriving later leaves no time for a leisurely cold one before supper, and makes the whole experience too much like work.
