A request is just that--a request. If you get what you want, great. If not, you can go to "dining changes" after you board and ask to be changed. Again, they will try but there is no promise. Sometimes they can do it as you stand in line. Sometimes, they will place you on a waiting list and call you if it is available before the dinner seating.
Personally, I think people make too big a deal over which rotation they have. On the 7 day, it is nice to be in Lumiere's on formal or semi-formal night. SInce Pirate night has replaced tropicali and mexicali, PC is less of a deal...still the most "pirate like' of the restaurants. But, in reality, it doesn't make much difference where you are on which night. I'd rather request my favorite server, regardless of which rotation he is on.