Is this true or not?

Simple question: Does it harm your electrical system (1974 Duster, totally stock in my case) to leave the ignition switch in the "run" position for any length of time with the engine off? When I want to listen to the radio with the engine shut down, I move the ignition switch to the "accessory" position and everything works just fine. But my buddy says that to listen to the radio with the ignition switch in "run" with the engine off will damage the electrical system. The owner's manual doesn't address this issue. And why would it damage the electrical system with the engine not running, but not harm anything when the engine is running? Ideas, anyone? Thanks - cruiser

Just put your radio on a switch so you have power to it constantly.
That way when the switch is on you can listen with zero draw from anything else.
And when the switch is off it’s definitely off.

I put one right next to the lighter in the ashtray compartment.
It switches the stereo and the amplified antenna.