A long, long time ago I gave the mirror of my Olympus OM4 a scrub and was horrified to find millions of tiny scratches on the surface (I used a piece of tissue!!). This crazing (circular swirls and scratches) was sufficient to befuddle the meter of that particular camera and the mirror was subsequently replaced for $460 (1984 price) with $180 labour. The repair job came with the advice "never, ever touch or put anything on the mirror" and said those mirrors are front-silvered, so extremely easy to scratch even with the finest microfibre cloths. I have not touched any mirror since that early lesson in DIY.