WR outlets; is it trying to tell you which way is up?
The NEC book does not address the issue, though many Outlet Suppliers have cut-sheets that say to mount with the ground up. From the brief comments I had with Electricians and Inspectors, it because if a plug is pulled slightly out form a outlet, there is a potential for something to connect the Hot/Neutral together by landing across the two spades. If the Ground is up, that potential for that is dramatically decreased.   What does the cutsheet for your outlet say?

Most plug (not outlet) vendors make plugs for devices that have the ground down as that seems to be the way most people install them.

All of my Installs have been with the Ground up.