We have done it both ways. First time, we booked a Cat. 8 GTY and ended up in a Cat. 6!! This time, we booked really early and were able to snag one of the SPH rooms so we selected our stateroom number! I would always select my room number unless I was booking a GTY status. I believe the system does not MAKE you pick a room - you can just pick a category and it will assign you the "best available". In this case, I think the system generates rooms starting with the center of the ship (if it's a category that has rooms midship) since those are generally better for less motion and then it works itself out to either end with whatever is available. Of course Cat 7's are only AFT and Cat 8's are only FWD so I think it starts as far midship as it can and works out from that...