When this subject comes up, there's usually always a few that will repeat the mantra: "never clean your coins".
No doubt such advice is born out horror stories of persons "cleaning grandma's gold coins with ajax" etc... But truth be told, some of the stuff we md'rs find (after being in the ground), will get laughed out of any coin store, if left in the conditions we find them in. So the real issue is, not whether or not to clean them .... but HOW to clean them.
There's the true story of a fellow on the east coast, who ..... after many years of md'ing, had a good handful of large cents. He went through the coin-book, and determined that some of them should have numismatic value. So he gathered up the coins he thought should have value, and took them into a coin store. The man at the counter looked at each one with his loops, and went through the coin book for each one. And on each one, he offered the man a pittance. When the md'r objected, and tried to show the coin-dealer that the coins should be valued higher, based on the detail criteria, mintage amounts, the dealer simply told him "yeah, but these coins are cruddy and dirty".
When the md'r heard that, he said "oh, then so if I clean them, THEN will you offer more?". When the dealer heard the word "clean", he BRISTLED and sternly warned the md'r: "If you clean them, then for SURE we will never buy them, since it's an absolute no-no to EVER clean coins".
Sad and dejected the md'r returned home with his large cents. He decided that since they were worth very little, he could at least display them in his own home collection trays, for his own purposes. And figured since they weren't worth much, then he had little-to-loose by cleaning them, for his own displays. He set about studying all the different methods, to arrive at the least intrusive least evident methods. He practiced on lesser valued large cents and IH's, till he finally arrived at a method he liked. I forget the actual method he used.
After about a year passed, one day, he was admiring his coins, and decided to take them in to the same store, to see what would happen. As he arrived, he recognized that the same man, as the year before, was working the counter. But the man apparently didn't remember the md'r from the year-earlier meeting. Without reminding the coin-dealer, instead, he just put the coins on the counter, and asked if the dealer was interested. The dealer, again, took them one-by-one with his loop and coin book. THIS TIME, the dealer made offers much closer to the book prices. Doh!
So you see, the old addage of "never clean your coins" is not always correct or true. It's HOW you clean them.