I usually add 10%. That's it. I know it's insane but I order room service from multiple Marriotts more often than not due to business travel. It's not worth it to me to make a "statement" as I will probably be back to the exact hotel and order again and probably get the same delivery person. I guess the sting is a little less painful as my company ends up paying it. Like it or not (and I do NOT) it is the custom we've created in the good ole' U.S. of A. To underscore that point - an Austrailian couple was in the elevator with me this morning (yes, I am in a Marriott as I write this) and they were laughing on how frequent they feel they must tip. I wanted to add something to their conversation but I did not. I could only share my frustration in quiet.