If the Room Steward has fulfilled all their obligations as far as cleaning your stateroom, etc., you are under no obligation to leave an extra tip. However, if they have extended themselves to provide you with services above and beyond their required duties, then $20 is a generous thank you. On my last cruise, my Room Steward never replenished the toilet paper, ice, soap, didn't make up our room until after Noon, never addressed our plumbing problems with Maintenance, etc., one of the worst Room Stewards I've had. On our last night, she approached me and asked if I would please leave her something extra. My roommate felt guilty and left her something extra but I did not. The Room Steward's lackluster performance did not deserve an extra tip.