I agree with arwenmark. It depends on what you order. For a "regular" room service breakfast we tip $3.00. If you order a complete meal, tip more. We consider ourselves good tippers, but would never tip $10.00 for a "regular" order. Something special, yes, but not for a normal breakfast order.
As arwenmark said, often the room service personnel are off duty from another job, looking to add some more to their income. So I suggest you tip whatever you are comfortable with.
We've never had our room service delivered by our room steward. We've always had wonderful service from the room service personnel.