I do - a buck or 2.

As a person who used to have to do this, I say Tom is right. Whenever people threw me a few bucks I really appreciated it. (It IS taking time away from the tipping folks and it is a bit of a pain in the ass, but it is indeed part of their job.) You don't HAVE to tip, but if you're asking and you'd like to be nice to the person who only works for tips, a few dollars would be really nice. I'm not talking 20%, but just an acknowledgement tip. 3-5 bucks maybe? Depends on how big the order is.