I'm at the IAH Term E PC now (Man is it Beautiful!!) and figured I'd simply ask an agent. Talked to Jo-Anne (a very informative Red-Coat) who told me that the agents should refuse any tips, they are doing their job. She did emphasize that a comment card describing the exceptional service would be MUCH more appreciated than any monetary token of appreciation as it "goes way up to the top".

Tips to the barstaff are welcome (obviously), and she didn't see anything wrong with tipping the bussers (my word, not hers), she had not seen it done and she noted that they are probably paid a little better than the bartenders (in keeping with the tradition of US waitstaff policies).

On another note, I tended bar for many years through college in both public bars and private country clubs. In all cases the base salary was $2.13/hr, plus tips. Even though the club is private, the assumption is that in the private clubs, your clientelle is going to have a little more cash and tip better. What the patrons don't often see are the hours spent setting up, tearing down and cleaning that the staff is still doing for $2.13/hr, no tips.

Hope this helps with the original question, forgive my 2 cents about the ensuing discussion.