In Shenandoah, if you will be showing up on the weekend, you may need reservations. If you show up on Sunday afternoon, you will probably have many sites to chose from, as there are many walk-in sites there.
I highly recommend Ricketts Glen State Park in PA. The waterfalls are absolutely beautiful with fall color.
Typically, I look at the reservation sites when I decide I will be going in that direction. If there are already few sites left, I will make a reservation. Or, if I know I 'must have' a particular site, I will reserve it (Site 180 in big meadows, for example) any chance I can. If there are plenty of sites available, I will wait until my plans firm up or I see that the sites are filling.
