I realize this is an older thread, but maybe someone is reading this for the first time like I was :-)
Checking with friends who cruise frequently, she suggested the following.
Pay the "recommended" amount that is billed to your account. In addition she recommends leaving a dollar per cabin guest on the pillow every day for the people caring for your room. She also left an envelope with a tip for the waiter who served you all week. Also remember those loading your luggage are usually not employees of the ship, so giving them a $1 per bag might help ensure you get your luggage safely to your room.