The NEC does not have a requirement on this, however you must install all equipment according to any instructions included with the ilistingor labelling. If the manufacturer had their equipment listed with ground up or ground down, then you must install it that way. Also remember that no equipment (listed or not) is approved unless it is acceptable to the Authority Having Jurisdiction (AHJ). The AHJ may require that receptacles be installed with a certain orientation.
Nothing is easy.