Yes, it is customary to tip a few dollars when room service delivers. We tipped $2 or $3 for breakfast or snacks, would probably tip a bit more for dinner.
dorothy
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It is true: Tipping is discretionary, so if you receive bad service you can elect to give no tip whatsoever. Many things are optional aboardship, including tipping, the dress-code, waiting in queues, not trying to reserve chaise lounges, etc. The decision whether to comply or refuse to comply is up to each passenger.
If you decide to disregard the cruiselines stated policy that tipping is customary, then you have the option to do so, but tipping isn't "included" on the mass market lines.
dorothy
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Room service IS included in the cost of a cruise. There is no policy which mandates tipping for room service. Sorry it is not my custom nor my practice.
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True, sort-of, though, based on his later reply, it is evidently not what the original poster meant with his question, and folks should feel good about having provided a more complete answer than that one, which only addresses the question very narrowly.