Most dim sum restaurants have a "tea" charge per person and this traditionally is considered part of the tips for the dim sum servers collectively since most other restaurants do not charge for tea. (I know some do now.) I usually pay the tea charges plus what I normally pay for gratuities. Many others deduct the tea charges from their tips.

I have never tipped the individual servers though but I am sure you can if you receive exceptional service. They will find a way to stow the money.