While any gratuity is always at the discretion of the client, it is customary in many places. The fee you pay for your massage to the therapist or cashier belongs strictly to the business, and the therapist may only receive a percentage of what you pay. While tipping is not required, it is most welcome and goes directly to your massage therapist whether the therapist is the owner, partner or employee of the massage business.
Tipping the person who gave you your massage is a nice gesture if he/she has given you a good massage. If you do decide to give your therapist a tip, the amount to give is entirely up to you. Like in restaurants, average tip for good service is 15-20% of the charge for the particular service the therapist performed.