What machka said, book something for the night you fly into a place and the night before you fly out. If you're headed for a really popular spot where you expect a crowd, booking a place as far ahead as you are able will save you a lot of aggravation and money. At a lot of small hostels, hotels or campgrounds, the owner or staff will pack up and go home if business is slow. A phone call at midday if you expect to be arriving late can ensure that you'll have a place to crash.
A lot depends on where you're touring. Off the beaten track you can get away with a lot less planning. Stealth camping isn't tolerated or easy everywhere, though, and if the last cheap bed or campsite is taken in a big city or popular tourist area then you can find yourself forced to pay a much higher price than you planned on for accommodation.