Only once in several years has anyone knocked on our door when the “Room Occupied” sign is up, and when that happened, I asked them not to do it again, and they never did. Once in a while they have called our room to ask what a good time would be, but mostly they have just worked around our schedule, based upon when we have the sign up. Even if they do knock, I have never once heard of them barging into your room against your wishes, as opposed to coming back at a time that is convenient for you. Anecdotally, I get the impression that if you are a known, repeat guest, they will be more relaxed about the checks than if you are a brand new visitor.