If it's been in the ground for a while, you might as well clean it, since it's already lost any numismatic premium. Just be gentle. Mild soap and water works ok. Olive oil works, too. Ammonia is by far the best for all coins except pennies. Coin dip is phenomenal, but only dip for a half second or so until you have practice. That stuff it powerful! For pennies, try vinegar and salt if olive oil or soap doesn't work. The most overlooked part of cleaning coins is the rinsing and drying. Rinse them thoroughly with water, and very gently rub them dry with a soft, lint-free cloth.
If it hasn't been in the ground, don't clean it unless you are willing to destroy its value.