I always thought that room service is not FREE. Isn't it paid for when you make that last payment?
As for the tipping. Everybody either eats their meals in the dining room, or buffet, but everybody eats, so everybody automatically tips, but not with room service. I've never ordered room service, and although it's there for the taking (so to speak) I would hate to tip for a service I didn't use. IMHO I think that it's easier just to tip whoever is bringing you room service at the moment service is rendered.