there was a treasure magazine article , decades ago (before the internet and forums), where a guy wrote an interesting article on this pro/con subject :
The east coast guy had amassed many dozens of LC's and colonial coppers over the years. One day he sat down with a coin-book in-hand, and studied the dates and conditions of all of them. Lo & behold, 5 or 6 of them he determined should be worth some collector money !
So he took them to a local coin store in his town, and laid them on the counter. The proprietor went through them, coin by coin, and offered a mere pittance for each of them. The md'r objected and tried to point out that the coins were better dates, details showing, etc.... But the coin-store guy said : "Yeah, but these are cruddy and dirty".
To which the md'r said : "Oh, then if I clean them, then will you offer more ?" . The coin store guy BRISTLED with disdain saying "Never never never clean your coins. If you did THAT, then we would CERTAINLY never buy them".
The dejected md'r left the store with his coins
And now he figured that , shucks, since these aren't worth much, I might as well clean them so that they at least look good for my own home display trays. And he set about studying all the methods . He experimented on some of his common ones, and ...... after a month or two, settled on a particular method that he felt left the least trace of effort. I forget what method he chose.
A year went by and , one day he decided to go back in to the same coin store with the same 5 or 6 coins. When he walked in, he recognized that it was the same man that had waited on him a year before. But the clerk did not recognize the md'r man. He puts the same coins on the counter. And ..... one by one, as the clerk checked his price guides, he offered considerably more than he had the year before.
Moral of the story ? It depends on how you do it. How good you are at leaving no trace of you effort. But it is not true to say that you should NEVER clean your coins. Because, let's be honest : Some of the stuff we find will get you laughed out of the coin store. So we are often-time in a position where it doesn't hurt to try, in some cases.