If you use information from NEMA as part of the listing and labeling requirements, and guidance on 110.12 matters, they are even particular about which threaded hole you use. You shouldn't, for instance, use an unused clamp screw hole for your ground screw. They even prefer that the ground screw thread pitch should be 10-32.
From the NEMA document, "Selection and Installation Guidelines for Electrical Outlet Boxes":
2.5.2 Grounding and Bonding
2.5.2.1 Metal BoxesThe NEC? requires that metal boxes be
grounded independently of any devices installed.
This may be accomplished through the use of a
continuous grounded raceway or cable armor, or
through the use of separate grounding
conductors. If grounding conductors are used,
the following methods may be employed to
ground the box. If multiple grounding conductors
are present they must be spliced together before
attaching to the box.
1) Attach the conductor to the box using a 10-32
ground screw. This is the preferred method.
Whenever there is paint or corrosion on the
box surface, under a screw head, the area
should be scraped or sanded shiny before the
ground wire is wrapped around the screw.
Other holes, including tapped holes containing
other screws, are not intended for grounding.
2) Use a listed ground clip on the side of the box.
Such clips are only suitable for use on flat
surfaces. Ground clips are listed for the
thicknesses of boxes for which they have been
evaluated. If the clips have been evaluated for
a range of thicknesses, the package of clips
will be marked accordingly
