Tipping is always appreciated and how much can be confusing.  If the massage therapist works at a major chain then typically they a page less an hourly wage and usually under $20 an hour (near Philadelphia areas) so a tip compensates their work.  If the person works for a medical office (near Philadelphia areas) their hourly wage is much better.  The best thing to do is simple ask the owner.  A good one will let you know how much they are making an hour so you can decided based upon your financial needs and theirs.