I don't believe it is a requirement except in a substation type enclosure. 250.194 2014
I also do not believe NFPA 780 requires it.
We do ground fences for the military. Here is an excerpt from a typical mil spec as it relates:
3.4 FENCES
Except as indicated below, metal fences that are electrically continuous
with metal posts extending at least 2 feet into the ground require no
additional grounding. Other fences shall be grounded on each side of every
gate. Fences shall be grounded by means of ground rods every 1000 to 1500
feet of length when fences are located in isolated places, and every 500 to
750 feet when in proximity ( 100 feet or less) to public roads, highways,
and buildings. The connection to ground shall be made from the post where
it is of metal and is electrically continuous with the fencing. All metal
fences shall be grounded at or near points crossed by overhead lines in
excess of 600 volts and at distances not exceeding 150 feet on each side of
line crossings.