10 seasons & something like 1500 nights behind us, we seldom make reservations. That is not to say that reservations are not made. Much depends on the where & when we are.
Sunday to Thursday reservations are very seldom needed anywhere. Even weekends one can often wing it. The trick is to get where you want Thursday or early Friday. Seems that most CGs will have a few non reservable sites.
All bets are off if you are rolling into an extremely popular area or when there is a special event going on. That is when booking way ahead becomes a necessity.
The FHUs are always going to go first, then the W/Es, then the long dry ones, finally the small dry sites. It helps not to be fussy & flexible.