Old wives are pretty smart sometimes. It is absolutely true, allowing "cold' (ambient) air to get to hot exhaust valves (the "open" ones) WILL warp them. I have seen a Harley valve bend so bad it broke when he tried to fire the engine again. It makes perfect sense. The air approaches the hot valve from one direction, cooling one side much more rapidly than the other.
When we "run engines in", and use only the manifolds, each of us are ready to stuff a shop towel into the manifold immediately upon shut-down. I would never consider firing an engine without at least manifolds.