On some cables the ground wire is only connected on one terminated end. It helps to reduce ground noise or hum. On a power cable that is grounded or has a ground plug it should not matter that much as long as both ends are terminated and your equipment that is using the cable is utilizing the ground. If it not using the ground then we might run into some ground noise issues because the signal is still there. If your piece of equipment does not have a ground ( like on a receiver) then it can cause ground loop hum. In this case the wire is not terminated or only terminated on one end. Also I have heard if you have a very sensitive system then the way the copper is extruded determines the direction of the signal.
power cord directionality
IF power cords do indeed have preferred directionality, and one is inadvertently flipped for a few hours, does it "harm"it?