You can find the criteria used to accurately grade U.S. coins in the Official A.N.A. Grading Standards for United States Coins. It's wise to remember that 3rd-party grading services exist specifically to provide unbiased grading and authentication services. A list of these companies can be found in our Grading Services search page. Additionally, you may wish to read our Grading U.S. Coins article. Generally speaking, any type of cleaning will lower a coin's desirability to collectors.
Humidity is another enemy.
Just one fingerprint can end up costing you a lot of money in the long run. Because of home security reasons, it may be best to use a post office box or work address for your coin collecting subscriptions and the like. A sad fact, but better to be safe than sorry. You're probably already doing this with credit card and other sensitive materials anyway. Some areas of the rare coin market have been outperforming stocks and bonds.