Regarding copper coins, aquachigger on YouTube has a real good video on options for various cleaning methods, along with pros and cons for each method. Here is link to that video.
https://youtu.be/w7cfVkadI_A
Regarding silver coins, if they are heavily corroded I don't hesitate to clean them. I fully realize that cleaning them can decimate the numismatic value, but I'm in this hobby to enjoy my discoveries in my display case, and not to sell them. Everyone is different though.
Here is an example that I posted a couple of months ago when I had initially found it. It was a Barber dime that was so fouled up that I couldn't see the date so I decided to clean it. It ended up being a key date, but I am 100% happier with it after I finished working on it, even though the monetary value will be low as a result.
Here it is pretty much as it came out of the ground.
This is after I aggressively cleaned it.
Then I artificially toned it.
And finally I worked on it to bring it to a more natural look.
Again, everyone has a different opinion when it comes to cleaning coins and there are many, many threads on Tnet discussing it. But personally, I'd rather have a pleasant looking coin than an unrecognizable metal disk.