Any of the above are deemed suitable; they are to allow for slight imperfections on the outer faces of the wheels & equalise the loading of the metal discs. As with all grinding wheels; apart from resin bonded type give them a 'ring test' with any non metallic tool, screwdriver handle, small hammer shaft etc, if it rings your good to go, if a dull sound, bin it, as it will have a crack/damage somewhere.
George.