Which model Dash? IIRC, the AFM of the -100, -200, and -300 series stated that only the pilot flying was required to wear the shoulder harness. In the -400 it states that both pilots must wear the shoulder harness below 10,000' and the PF must wear it with the autopilot engaged. The operator may also put specific verbiage in their SOP that states pilots shall wear the harnesses at different times that is above and beyond that described in the AFM.
Outside of the limitations, it comes down to pilot preference. I see some guys ditch the harness as soon as we pass through 10,000' in the climb and put it on as we pass 10,000' on the way down, regardless of flight duties, while others will wear it the entire flight.