You must fly the aircraft in accordance with the Flying Manual/Pilot's Operating Handbook.
Some FM/POHs state that the aircraft must be flown solo from the rear seat for CofG purposes eg, Piper Cub. However, I've never seen a FM/POH which states the aircraft must be flown solo from the left seat.
Group Rules or a Flying Club Order Book may require the left-hand seat to be occupied for take-off and landing. This makes things less complicated with the insurance company following an accident/incident.