I never book B&B, I have found over the years its just as easy towards late afternoon to keep a eye open for the vacancy signs, On the few occasions I been told we only do two day bookings, i said that OK, I have seen other B&B down the road. Which has brought a change of mind, as one nights money is better than none at all. They know that generally if they have not had a weekend booking by late afternoon the chances are that they will not get one, and need to hope for passing trade. I do understand in the Wilds of Scotland where B&B can be in short supply one should book.