Bonding bushing should be used anytime a metal conduit enters any type of enclosure when the conductivity can not be ensured across where the conduit and enclosure meet. One example is a cocked conduit or a conduit is not square to the enclosure. this will prevent the locknut(s) from mating squarely to the box. Another example is the concentric / eccentric knockouts. Although they are usually rated for carry fault current, some times they limited on the rating or the concentric / eccentric sometimes get banged around during installation. When in doubt, throw on a bonding bushing.
