The following is from:
http://www.mycoincollecting.com
The first rule of coin cleaning is: If you don't know its value, don't clean it, or if you think it is valuable, don't clean it, or if you know it is valuable, don't clean it. In other words, do not clean your discovered coins; leave them untouched and stored in proper holders. The reason for this is that coin dealers and collectors are interested in purchasing coins in their original condition and natural state of preservation.
For example, never clean coins with commercial jewelry and metal polishes or silver tarnish remover, which will remove the toning that normally collects over time on copper and silver coins. Removing tarnish often harms coins, leaving small spots, scratches, or pockmarks that can significantly diminish their numismatic value by up to 90 percent! Statistically, nearly 20 percent of coins shown to coin dealers are rejected because they were improperly cleaned, polished, or their toning was chemically enhanced. Remember, you cannot unclean an improperly cleaned coin.