I have looked at the schematic diagrams for nearly every Yamaha arranger keyboard made, up to the Tyros 5 and PSR-S950. The output connections are identical. When you place a 1/4-inch plug into the L/R mono output jack it triggers a latch switch on that jack that automatically connects the right and left outputs together. The Yamaha rep that said you could plug into either and get the same results doesn't know what he is talking about. If you were to plug into just the R output jack, you would only get what comes from that channel.
Hope this helps,
Gary