We all know that vehicles get covered in salt in the winter. Do you coat your vehicle in mud afterwards so that the salt wont rust the body?
Non corrosive will not clean out or burn off all of the salts in the barrel and gas system.
Umm... No... Think about it for a bit.
First off, the fouling gets deposited in layers and builds up over time. The fouling from the first thing you shoot will be closest to the metal, and the last thing you shoot will be deposited "on top."
I've heard the rumour before, but like the others have said, doesn't make much sense when you think about it. On the flip side, I would never trust it to not rust, but it makes more sense to me to shoot non corrosive first, then the built up carbon layer may provide some sort of barrier between the metal and the corrosive material. I wouldn't do this thinking you don't need to clean the gun afterword, but it makes more sense than firing a few rounds afterword.