I tried showing up the day of a tour and asking about last minute no show spots and was told they do not have a waiting list for those spots, nor is it policy to allow others to take the no show spots, because they were sold to someone else. I assume that meant if a party was a bit late for the tour, they might be taken to catch up with the rest of the group. The CM also said it is very, very rare for people not to show up for tours, especially the holiday tours. That doesn't mean that the CMs don't have the ability to let you have a no show's spot, but don't count on it.
FYI: When I finally was able to make a tour reservation (after several attempts) I was told the tours for Halloween had been selling out within 30 minutes of the office's opening, so keep that in mind for Christmas.