I used to rock up to towns in Europe wihtout reservations, and to be honest it is never a good idea. I was young and rather stupid at the time, and sleeping outside didn`t bother me much. I always took my chances. Sometimes I got a room, sometimes I didn`t.

If you really want to have a nice relaxing time without stress I would definitely reserve in advance.

One time I went to Spain thinking I would get a room easily enough and it turned out to be during the running of the bulls, and getting a room proved to be impossible. You never know what will be going on.