Generally they won't do this if the Do Not Disturb sign is out, so I always put it out. On the other hand, there seems to be an actual policy at some properties (including the HR Denver Downtown) that if the DND sign is on and it gets to be a certain time, they will call you on the phone to see if you want service for the day. I find this to be unacceptable, but the property disagrees (although they made a note on my profile not to do it to me again). Other properties slip a note under the door, which I find to be acceptable.