I, too, push a full load of Star San out from one keg to the next, but then I also hook up to the liquid out post and let CO2 run for 15-30 seconds. Once I start racking, I hook up to the gas in post and let CO2 flow at a slow rate for the duration of the transfer. Obviously a flare connection for the QD is useful.
It's true that CO2 is heavier than air, but don't think that you can really stratify the gases over any significant period of time. They'll mix and mingle pretty quickly, which is why I keep the flow of fresh CO2 going. And you might be surprised at how ineffective pushing liquid out of a container is at purging it of O2; I was. I thought if I had a container completely packed with liquid, pushed it out with CO2, and kept it sealed the whole time, it should remove all the O2, basically, but not so much. There's always little pockets or bubbles. That's why I also purge after I rack the Star San out.