I've traveled in the PNW quite a bit and have never made reservations and have never had trouble getting a spot. Now, I am a tad more careful on weekends - but still go without reservations because I never know exactly where I'll be - except when meeting up with friends. It's like the FL state parks in the winter - weekends can really fill up. And check with your parks - some of them hold a couple sites for walkins - I don't remember if they did up there or not.
Now, holiday weekends - worse than weekends, things fill up. I try to figure out where I'll be staying for those times - or to have backup plans. To do a gtg on a weekend, having sites is good - cuz where one person might be able to get a site, several might not.
That said, it sounds like there will be several of you around, I suggest someone work with Liz and Sharon - if they can't be there at the same time, then set a couple different dates at different parks - and make a committed gtg for one or both of them = and see who else can come. More will come if it's a done deal and the decision is made for them
(Right Liz? - she can tell you about the FL gtg that never happened as it was discussed to death - the next year, she picked a date and time - and voila, people came.)
Love that area - so many great places to go - inland and by the water.