I'm constantly on small aircrafts (CR2s and ERJs) and for weight and balance the pilot always ask for some volunteers from the first three rows to go sit at the back of the plane next to the toilet. No other choice, it's always the seats next to the toilet.

One time on my flight EDI-EWR, a passenger from Economy decided after 2 hours to come sit down in Row 20 (one of the Exit rows) which is in Economy+. After 30 minutes one of the FAs told him to go back to his seat.