Operating a helicopter with rotors running whether it be on the ground or airborne and not wearing a harness/seatbelt is about as stupid as riding a motorbike without a crash helmet. There are also the legal implications.
Article 50(4) ANO : Each pilot at the controls shall be secured in his seat by either a safety belt with or without one diagonal shoulder strap or a safety harness....................
The normal pre-start procedures in an aircraft flight manual for a helicopter will include an entry similar to "Seatbelts - Secure/Fastened". The flight manual forms part of the Certificate of Airworthiness. If you operate outside of the procedures and limitations in the flight manual you also are compromising the Certificate of Insurance as well.
It's a no brainer. If you're going to start a helicopter, even if only to carry out a ground run, the belts/harnesses should go on.