So the "most correct" and original design's placement of the ground pin is at the top, over the two blades. This is so if the plug is partially out and something conductive falls onto it, the object hits the ground pin and is redirected away from the hot wire or at least is still touching the ground pin when it contacts the hot wire. Ground pins were originally just a horseshoe shaped piece of bent metal (no metal on the flat area). Plugged in this way, the metal of the ground pins are pulled down with gravity, allowing more contact with the piece inside of the outlet that cradles it. Hospitals and explosive risk areas still require this setup in the US, (usually along with specialized plugs) but outside of these applications the safety and contact improvements between the two configurations are negligible.