I've certainly never heard of people's own weights being requested, though logically perhaps they should. I daresay they trust to the pilot having a good eye when he looks at who and what are boarding his aircraft.
I've actually been in a Caravan (the normal aircraft here) when all five other passengers were seated as far back as they could go. I was surprised the pilot didn't ask them to move forward. There seemed to be no takeoff or landing issues, so I deduce that balance is not a critical matter for these aircraft.