All compass calibrations should be done with everything on that is normally on in flight, including headsets, if you want complete accuracy and compliance with the rules & commonsense
The manual says that the aircraft should be in flying attitude while calibrating the heading. A pain for a taildragger! How much error would be introduced if a taildragger did not have the tail elevated? (I'm at about 35 deg S).
Which manual and rev do you see that in (we'll need to update the instructions) ? Basically, if the attitude is showing the correct nose-up attitude, calibrating on the ground will work fine.