If you'd like a specific room, deck, or location on the ship (i.e. deck 7 aft, etc.) you can ask for it in the box where you list the names of everyone in your party and any special requests, near the bottom of the "Get a quote" page.
If you want a specific room and it is open, then they will book it for you. If not, they'll try to get you something near the room you wanted. When your quote is returned, it should list a room number (it won't if the only thing available was a category guarantee). The room number isn't etched in stone, and they can change it for you, based on availability.