No. Tipping the girls is optional. When you are given a price quote, this is inclusive of the complete service. In case you feel like, you can tip the girls some extra money. The girls might ask you for a tip, however, this doesn't mean you have to tip them. Most of the time, you tip a girl only if she has given you the best soapy massage Bangkok experience. A decent tip would be about 300 baht plus, at the soapy massage parlors.