We have done the NW a couple of times now. Both times in the early summer of May, June. Both times no reservations. Both times no real problems getting into CGs.
Weekdays getting a site is easy, even in the really nice SPs. All seem to hold a few sites for drive ins. The trick is to get there no later than mid afternoon. Also for weekends, get there on friday & stay till sunday.
FHU & water/electric sites go first. Always be prepared to drycamp for a few days. Much less demand for those spots.