I'm no pro, but I do have some experience. The 'cheat' seems reasonable to me in this situation. It depends on how wide the cracks are and the condition of the cracked grout. Thin cracks would take non-sanded grout with the closest color match. Thick cracks would take sanded grout and the color difference may be more noticable. In any case, I'd check the condition of the grout by running a grout saw lightly over it to see if it crumbles easily. It's not a good idea to patch over badly weekened grout.