welcome to Cruise-Chat.com.
What do you mean by "rear room"? A cabin near the stern of the ship on either starboard or portside or a cabin directly overlooking the stern?
I wouldn't consider either cabin a bad choice. While usually, the lower in the ship and the more towards midships, the less a ship's movement is felt, on large ships such as the "Carnival Conquest" you usually don't feel a lot of motion, unless the seas are fairly heavy.
If you have a balcony cabin overlooking the stern: congratulations - those are excellent. I had one on the "Carnival Liberty", a sister ship to the "Carnival Conquest". The cabins are identical to those on starboard and portside, but the balcones are considerably larger and personally, I just love watching the ship's wake.