When I was on board, I asked both the headwaiter (Stephanie) and Guest Services about making dining rotation requests as I did not get what I thought I wanted. Both told me that they really do not honor them...there is no way that they can do that when trying to do seating for 2500 people every week. (After doing my wedding, I think I can empathise, and I only had 150 people!!) They said that if someone truly is dissatisfied with their rotation, it is best handled on board. Bottom line though, is that I thought I wanted a certain rotation based on my research on these boards, but once on the ship, it did not matter at all. We had formal night in Parrot Cay and it was great!
With respect to being assured of sitting with other members of your traveling party, you can link your reservations together and then you will be all but guaranteed to be seated together.