been there - done that
Crossed back and forth plenty of times and have had no problems at all finding rooms, but if you are really picky, you may have a problem once or twice in high season popular destinations. Yellowstone & Yosemite, The Outer Banks and Key West come to mind.
I don't care to be locked into a hard schedule, so I avoid reservations more than a day or two ahead for the most popular places. I've often found some nice places that I never would have discovered had I been locked in to a reservation.