I know this topic comes up once and a while, but I just came back from CN, and saw a few people giving tips to the bartenders. For those who do leave tips, please remember that even if you try to leave them discreetly, it can be seen by others.
As Randymon says - if staff are caught accepting tips, they can be fired.
One of the pleasures I like about Couples is not having to tip. If you want to recognize a staff member, make a comment about them on your comment card when you leave, or bring them a small gift.