We went during the summer of 2011, Because we have a pretty big rig, we felt better booking our entire trip as far in advance (up to 1 year) as possible.
Mid week walk-ins in normal campgrounds would have been is pretty easy without reservations, but national parks (Yosemite, Yellowstone, Grand Canyon, etc.) would have been all but impossible.
Even trying as best we could to book Yosemite 1 year to the minute in advance it still sold out and we had to camp just outside the park.
See our cross country trip link in my signature.