For any procedure such as a tattoo (the dentist, endoscope, colonoscopy, etc, etc) you should stop taking your coumadin five days in advance. Notify the whoever monitors your coumadin levels. The "coumadin girls" at my doc's office are very helpful and work with me on such stuff. When to restart is answered by the doctor, or tattoo artist predicated on their judgement as to when the bleeding will no long be "spontaneous." (normally the next day).
HOWEVER, a wise man (or woman) could check with their doctor FIRST.