I am a massage therapist and tipping is always optional. Often when a massage is given in a spa setting or for strictly relaxation purposes - tips are appropriate. These therapists are usually paid a 1/3 of the fee per hour and depend on tips. Clinical massage, meaning massage on a regular basis to correct a problematic area or work with an injury or in conjunction with other practioners - tipping is even less "assumed" (for lack of a better word!) and again, appreciative, but optional.
I know therapists who work for Massage Envy and they do definitely depend on tips. They are some of the lowest paid massage therapists out there - which is how the business can offer the deals they do.
There are no rules 10%, 15%...etc...just give what you can and based on the services you receive. In general $10-20 is typical, but I've never been ungrateful for a $5 tip and I've also been given even greater amounts on longer massages.
Massages are generally enjoyable experiences, so ask yourself - did the therapist communicate with me and meet my needs? Did they focus on areas that you specified and/or educate you as they went along if they needed to focus their time on other areas or short change special requests?
Enjoy your massage! It's a wonderful gift and offers so many health benefits!!