30 day trip out West - Dare we without reservations?

Some Yosemite NP CGs are first come, first served (no reservations). If you get to the CG on a Sunday morning or early in the week morning, you stand a good chance of getting a site. If you want to stay in the valley, though, reservations are the only way. This year, a couple of weeks before leaving home, I was able to pick up a Sat and Sun at 2 separate valley campsites... I had been checking on and off for months, though.