We typically specify a copper ground for everything. The only exception I can think of off-hand is if we're re-using an existing conduit and adding a ground wire would put us over 40% fill. In those cases, if the conduit is currently being used as an EGC and the Electrical Contractor tells us that he's comfortable with it (no evidence of breaks, high-resistance connections, etc.), then we'll go ahead and continue using the conduit as an EGC. If it's just an old spare conduit and we don't really know anything about it, we have the Electrical Contractor verify the continuity/low resistance path before approving its use as an EGC.