It is not a myth and is how the thermostat is designed to operate from the factory. Every single LS equipped car used either a 4 port heater valve or in most cases no heater valve and instead air is diverted in the heater box.
That said the engine will run fine you just might see slightly warmer temps at idle. At higher RPM's the bypass in the thermostat opens so hot engine coolant will flow across the back of the thermostat even if the heater port is blocked.
I made a video about the difference I saw in thermostat behavior with and without flow through the heater hose port.
Edit. The video is long and boring but what you see is that with flow I get a normal cycle of the tsat but with it blocked my car idle temp is 207ish.
http://youtu.be/oxg4dICnCwc