Because of the heat conduction of alu, the weld cools very quickly after stopping the weld. Most of the time it already cools in the slope down. This is enough to limit oxide forming. Watching prof alu welders doing their work. I notice that they don't spend much time on the post flow.
I use loads of post flow when welding ally. If your tungsten isn't nice and shiney after you have finished welding then you don't have enough post flow. I'd start at about 6 secs min, more if running big amps.