Generally room service is delivered by a room service waiter or sometimes by your cabin steward. If by the latter, you could wait to tip him at the end (I do not). I generally
take out 2 singles when I call room service and that is what I give to whoever brings the order. Like most other things, if you tip at time of service the next time you will get good service. Bar staff , by the way, are taken care of with 15% gratuity added to each tab.