In my experience, thay do not care if you show up with less people, but they do not like it if you show up with an extra. This happened on my birthday a few years back, last minute, and we switched from Askerhus to Garden Grill. Of course, this depends on the restaurant. I always call, or stop at Guest Services at my hotel, if I need to add a person. I have always been thanked for this. Seating at some restaurants is exact, and might save you a longer wait, if you show up with that extra guest.