NOT TRUE!!!
A rag is MUCH WORSE than nothing. Turns your gas tank into a Malotov cocktail, just waiting. Fumes will permeate the rag, the rag will cause more vapors to stay in the filler tube, concentrating them more than if they had an unobstructed path, and the rag will act like a catch-all for anything flammable. The rag will become an easier ignition site than just the fumes coming out the tank.