Like Mr Splock, I used to pamper my coins, forgetting their hard-working past. But now I do not worry over-much, as damaging them would not be easy with normal care and handling. Pieces with flat undamaged fields should not be handled in such a way as to add scratches, but beyond that, almost anything goes. In fact, regular handling of coppers tends to improve their appearance in many instances, by the application of a little sweat containing small amounts of fat/grease - which makes me wonder if rubbing with oil or butter might do some good to some coins? We all get dirt on our fingers by handling dirty coins, and that dirt is from the coin surface - if it is now on us, it is no longer on the coin, which must be good news? I venture to suggest that rubbing between finger and thumb tips has never damaged any ex-circulation coin (the only kind I buy), but cleaning products often have done so.
Salvete