I've been welding for ages. Never used post-flow with mig on steel. Post flow might be used if you were doing aluminum with a spool gun - can't say for sure, I tig aluminum. Pre and post flow is used on all tig welds.

I'd bet 99.99% of guys mig welding don't use post flow welding steel. They don't teach it and its not required for certification. The .01% using post flow probably don't know any better and are wasting gas.

It only been within the last 10 years or so that a mig has been available with pre and post flow. It was only when I modernized with a Miller 252 that I had the capability.