I don't pay much attention to washing the underside of such plates, and the washed plates don't seem to retain any noticeable dirt on the underside either. So I

I don't pay much attention to washing the underside of such plates, and the washed plates don't seem to retain any noticeable dirt on the underside either. So I'm inclined to think there's not much of an actual problem. Maybe if the household has that culture of not eating half of their portion, then there's a lot more excess food in play.