It depends on the place, but I will usually try when I checkin- sometimes before via telephone, if they say no, I may try to later on in the stay if the person who refused isn't around anymore at the FD. Everytime I have put my number in, it has gone through OK. One methos that I find works OK is to just say I want to put my airline number in, which I think works better than the hotel number, I always seem to get the miles.