My wife and I have a few specific extra things that we ask for on day one, e.g. place sheets over the sofa and chairs, extra ice, a hot water kettle for the room, etc. For this I normally give the room steward $20 at the time we make the request.
Also, if the room steward does more than just normal cleaning and servicing of the room during the course of the cruise, we trip again on the last night or the morning of the last day of the cruise.
We consider ourselves to be very low maintenance but if we see that any wait staff, bartenders, room stewards, that we encounter daily put forth a little extra effort, we feel like they should be rewarded.