I sometimes pre-booked if I was going to be late arriving, this was 30 years ago and there were plenty of hotels. Now as LWaB says, many areas are getting quite empty, with a town having perhaps one hotel instead of 2 or 3, some with none at all where in the past it wouldn't have been a problem. Food won't be an issue - even relatively small towns have supermarkets these days, but maybe not a hotel. So now, I'd pre-book. Oh and September might be conge annuel time for some hoteliers, and in wine regions the cheaper hosteleries might have pickers staying depending on the timing of the vend age, all situations I encountered from time to time.