It depends on how picky you are. If you want a large site with privacy with a good view, book as far in advance as you can. If you will take any spot anywhere as long as you have a place to park, you'll find something. But don't count on it being a 'great' location if it is on the weekend or a holiday.
We are currently on the Oregon Coast (it's early season) and are taking what we can get. On Monday through Wednesday, if the sites are first come first serve, it isn't a problem. But the best sites are either reserved or already taken, even on a Monday...
Also, the less hookups you need, the easier it is to get a site.