Hobbies And Interests
Personal Coin Collections
If you just intend to put your coins in a binder to keep as part of a collection with no intention to sell you probably should clean them to improve their appearance. To clean coins, use the soap and water method or a special coin cleaner. Rub the coins with your fingers or a cotton towel, rinse them off and dry them. Make sure they dry completely before putting them in holders.
Coins With Little Value
Although older and collectible, some coins still don't have much value and cleaning them will not make much of a difference in selling price. A good example is a Mercury dime from the 1940s in fine condition. This coin is only worth about $1 and you might have a better chance of selling it if it looks nicer after being cleaned.
Gold Coins
Some of the newer releases by the U.S. Mint are valued more for their gold content than their rarity. Therefore, if you happen to get some blemishes on yours it probably won't harm the value of the coin to clean it. Gold is unlikely to become discolored if you clean it because it is a noble metal---that is, it is resistant to oxidation. Use hot, soapy water and no chemicals for the cleaning process. Do not clean older gold coins as their value often comes as much from their rarity as their metal composition.
Valuable Coins
Coins that have great monetary and numismatical value that you may one day want to sell should not be cleaned. The luster created by the cleaning is frowned upon by collectors who appreciate the natural wear accumulated by coins after years of circulation.