Most places these days add an 18% to the bill already. Check and see if that is the case. It should say this on the room service menu.
Here is what I do:
If no added charge, I tip 20%
If charge is added, I may add 2 - 10 dollars depending on the total of the bill and what the server had to do. For example, if he/she has to set up a table, put on a cloth, set the table, etc, I will add a good bit. If they just set the tray on the desk and barely smile, I add nothing.