I do not always tip for room service. If our steward brings us something, I figure that's part of the service I'm tipping for at the end of the cruise. If I were to tip every time I received a service from a steward, I would be tipping all the time. For example, do you tip him for ice, making the beds or bringing extra towels? Now, if a girl from the galley brings the food (you can tell by the "uniform") then I will tip her because she is not part of the staff that gets a regular tip for service.
I know not everyone will agree, but that's how we do it.