Newbie here but my understanding is that, for the R22 at least, seatbelts MUST be fastened when in flight (seat occupied or not).
If the belt wasn't fastened and something went wrong it could then be shown that the pic was flying the aircraft having ignored a basic pre-start-up check, listed in the POH (it's certainly on the check cards at my FTO)... ...therefore aircraft in breach of CofA (of which the POH forms part) and therefore all sorts of ramifications... such as insurers able to duck out and leave you to pay for the consequences of any incident.
I'd rather not pay for anything I don't absolutely have to so I belt up. It's pretty much automatic for most uk drivers anyway now.
M.