If the staff at Cubist Pharmaceuticals Inc. looks a little more relaxed on Tuesday and Wednesday afternoons, it’s probably because that’s when the masseuse visits the Lexington company. Cubist treats its employees to a free 15-minute massage every quarter (30 minutes during the quarter in which their birthday falls) by a massage therapist who has been kneading tense back and neck muscles at the company for a decade. Each session takes place, fully clothed, in a room equipped with a massage table and soothing music. Those who work offsite aren’t forgotten, either: They can expense a $60 massage each quarter.Katie Johnston can be reached at email@example.com. Follow her on Twitter @ktkjohnston.