Let's do silver coins first, as this is where I see the most damage being done: If you find a silver coin (dated 1964 or earlier for most US coins) with your metal detector, with soil stuck on the surface of the coin, rubbing it in the field is as bad as cleaning it. The soil is VERY abrasive and leaves hairline scratches, the same as many harsh cleaning methods. Regardless of the condition of the coin, poor to mint state, soil rubbed across the surface will leave scratches that downgrade any potential value the coin may have. Don't rub silver coins fresh from the ground.
So you're saying to yourself, so then, Mr. Smarty Pants, how do I get the dirt off this coin so I
Still not clean enough? Mild liquid soap and a soft toothbrush (with a light touch) can be used, but be aware, this technique may damage the coin by causing scratches. If a grain of soil gets stuck in the bristles, as you brush and drag it over the surface it will leave fine scratches. I usually put a few drops of dish washing soap on a CLEAN soft toothbrush and tap at the surface while running it under the tap, rather than brushing across it. This loosens the dirt and washes it away rather than allowing it to scratch the coin.
Heavy green or black corrosion adhering to the surface can be removed with simple homemade electrolysis units. http://gometaldetecting.com/electrolysis_cleaning.htm Carefully applied, sometimes these coins can retain their value and not appear cleaned. This takes practice and careful steady observation of the process.
Commercially sold rust removers, such as CLR or coffee pot cleaners can remove rust stains on silver coins, but be aware these are mild acids and will etch (damage) the surfaces of coins. I leave rust stains on the coins I find, as I think it makes them more interesting.
So now that I've got the warnings out of the way, and feel confident you won't take your Dremel tool to that 1876 seated dime, there are a lot of coins where condition isn't that important. You just found a worn 1946 Roosevelt dime. Go ahead, rub it, clean it, buff it with silver polish if you must... it is a common coin and has little value over it's silver content.