On some cruise lines when you get room service they want you to sign a slip. They say that it is for accounting and service evaluation. In reality it is just a way to say that you should tip them because there is a line close to where you sign that says TIP. That is the only line they are really concerned about.
Yes there is a $10 gratuity recommended charge to your account everyday, but that is for normal service. Anything beyond that justifies more.
Is it worth going up to the buffet and back for a dollar? Or think about the employee that has to fill the pitcher and walk the length of the ship and a few decks for hopefully a dollar.
To me the biggest rip is the guy at the luggage check in that puts your bags into a container and DEMANDS a dollar or more per bag.