It's not a billionth of an inch...most inline sixes won't allow you to insert the cam on new cam bearings if it's hung from an engine stand.
Cranks can and do warp. I doubt that they develop "flat spots" from laying on the ground (if cast iron was that soft, the engine mount ears would eventually slide off!), but they'll certainly end up out of true.
I hang all my cranks (Leaving them standing up is just asking to have one knocked over. I'll lay it down before I risk knocking it over), and I store all my rotors/flywheels/etc flat.