Most all hotels/resorts offering room service began including the tip as a standard charge some years ago, as most persons were NOT tipping.
Therefore, most room service charges will include, a delivery charge (*for the property), state taxes, and a gratuity or "SERVICE CHARGE" for the server.
Of course, you may add additional gratuity if you feel so inclined (I usually do, as I find 18% insufficient for the preparation and delivery of such a special/convenient service. I also order things such as champagne, etc., which is usually opened and poured by the room service attendant. I tip cash whenever I do this, which is greatly appreciated.
I also tip on every "complimentary" service I receive. For example, I am flying Upper Class on VA airlines this Winter, RT from LAX to LHR. I just called the Premier service team reservation number, to schedule my complimetary/inclusive Limousine service to/from the airport. There is no limit on this service, which is amazing/wonderful (includes the entire STATE of Calif to the nearest Metro airport..and the same for all of London). I will be picked~up at my door, and delivered to the Virgin Area at LAX, likewise in London. I will be using it a total of four times, and will be sure to "tip" generously each time.

I sincerely appreciate the luxury/convenience such a service provides. Many people deem this a pre-paid part of the excessive price charged for such a level of service and/or class of travel. I, on the other hand, "appreciate" this service, and do not view it as merely an entitlement. The airline makes the money, and the person driving the limo "depends" on gratuity for additional income (sometimes VITAL income).
Hope this helps!
TTFN...DF99
