There is no reason to insulate the overhang. It will do nothing to increase comfort or lower energy costs, since it is beyond the living space. In most cases it is not even a good idea, since the overhang serves as part of the roof system ventilation. Since flat roofs are harder to ventilate than higher slope designs, you do not want to impede any venting that is present by closing it off with insulation. Any airflow across the underside of the roof surface is removing moisture that could condense there.