Adding grain is a bit like dithering in audio... an image that is overly perfect might be made more visually pleasing by a tiny, even unnoticeable amount of noise. It is possible that the grain is a sort of inherent dithering that makes film so pleasing on the eye. That said, if grain still exists in the OT sources, its hard to believe that artificial grain would do anything but degrade the image slightly.

Fake hairs, dirt, and scratches rarely look real, and I'd avoid them. Mouse droppings, however...