A true coin collector or dealer will tell you any cleaning will harm the coin and lower it's value. If it's a dug coin the damage has already been done by years of dirt, grit, rocks, fertilizer, minerals and heaving caused by freezing and thawing over and over. If it's a real valuable key date you can have it professional cleaned so you do not completely screw it up. If not, use baking soda, tumble it, scrub it, rub it, soak it in olive oil or vinegar or whatever else it takes until it's as good as you can get it and enjoy it for what it is!
Yup, Rob is correct. Any type of cleaning is wear on the coin but sometimes the result is better than leaving it alone. Sometimes the cleaning makes it worse. You have to take it on an individual basis depending on damage, coin value, and desired visual goal. I don't tumble anything that has collector value. Common coins can be tumbled but start with a mild media such as plastic shot or walnut shells. I like the walnut shells for copper. It gives them a nice burnished look to them. Clad just gets the aquarium gravel!