This is a GOOD question, and a DIFFICULT one for coins coming out of the ground. Modern coins of modern value can tumble, old circulated coins with possible value should never be cleaned in any style or risk devaluation, but for a dug coin with dirt/mud all over the surface to where its hardly identifiable will certainly need a clean to some extent, I understand your question. From my experience, to get that dirt off is different for each type of metal in the coin, and I have no perfect solution and am no pro. This is what I have heard/experienced if it helps, I am still always learning.
Some say the time some coins are exposed to air after being dug could cause them to dry out and cause damage, I have heard of some carrying olive oil with them to put old coins to soak to remove dirt, and others have said not to do that. Some say silver can be sprayed with a water bottle to get mud off but no rubbing at all silver will scratch so easily, and some say do not spray water on copper coins as it can flake the patina right off. Some say to put cold hydrogen peroxide on to soak for a bit and gently use a q-tip/toothpick to help get corrosion off. Some use products like coin care and renaissance wax to clean/preserve coins/finds that do not hold much value. Of course there is electrolysis as well. I have not tried electrolysis yet but I need to. There are a lot of techniques these old pro's have and I would love to hear more as well.
