Well, a metric value (1 unit = 1cm, 1 unit = 1m, etc.) will largely avoid rounding errors while an imperial scale (1 unit = 1 inch, 1 unit = 8 feet) won't. But as long as the instructions are clear, if you supply as OBJ, it doesn't matter; if you supply the items as pp2 or .duf then you will be doing the conversion on import and can choose whatever works for you.