water circulation question

I have a question on how water circulation works on our Mopars. The car is a 71 Dart non A/C. I am doing a radiator flush. Question--- do I need the heater ON to have warm water circulate through the heater core? I was under the impression since there's no shutoff valve on the heater lines that water is always circulating through the heater core in our cars. The only thing the controls do is open/close the air access vents.