Personally, I like having reservations. I tend to drive too long and having reservations makes me stop for the day. I also don't like the idea of having to spend time looking for a place to stay and being disappointed. After a long day of driving, I like knowing where I'm going to sleep. I also like to spend two or three nights in one place, which might be hard to do without booking in advance. Reservations also give me some assurance of getting.the type.of room I want,eg., a double with two beds. September is also a busy season, so vacancies might be limited. You could try calling ahead in the early afternoon. I often do that when traveling in the US, but I don't know how practical that would be in Europe.