If you aren't sure whether you feel a tip should be left, consider this: Do you know how much they are/aren't getting paid? Yes they are providing a health service. However, massage therapists can get paid in very different ways: employee wage, commission, or full price of massage and they pay rent, etc. Also, most of them (whom are self employed) pay their own taxes, so a tip can help to compensate for that.
Since there can be such a difference between getting paid minimum wage to work hard all day, while others can make the full amount of the massage, I say that you should. Its a nice compliment and makes a therapist feel appreciated. I do believe you should only do this IF it was good and you believe they worked hard though. You shouldn't do it out of obligation, but if you enjoyed the massage, then yes help them out.
I have clients who don't tip at all, some who do occasionally, and some who always do- all ranging from 0-40 dollars (the average being 10 for an hour).