I have found quite a few large coppers and there is not one that I would not have cleaned (meaning removing the dirt, not polishing or anything abrasive).
Already explained why, and the other reason is, to find out what it is in a lot of cases and the variety, one particular year coin might be a common one, but in the same year there are varieties worth hundreds and possibly into the thousands.
Even if not concerned about ever selling, then you are the collector and it is your responsibility to preserve the coin for the future to who ever inherits your collection.
Here is a reply I got from one of the top Colonial collectors/buyers and also a contributer to the Notre Dame website, he also has done some metal detecting in recent years with one of the top detectorists in New Jersey.
I asked him for his opinion on cleaning ground found coppers.
Hi Don. The mantra has always been don't clean a coin or it will lose value. For undug coins this is true. For dug coins this is untrue. The value will increase with judicious cleaning. The coin is the coin whether cleaned or not. The discussion I think centers on worsening the coins value or increasing it. If the value of the coin doesn't matter to you, then there is no need to clean a filthy coin other than to make out what the variety is. If you plan to sell the coin or to have bragging rights, I think some coins are made better with cleaning - that includes halting vertrigris. I am not knowledgable enough to say what is and is not a good method for cleaning.
To say not to clean a copper ,well, here are two examples of dirty coppers as found and with what dirt was left on them by the time I got home. Pretty hard to ID the coin in their uncleaned condition, but cleaning sure makes a big difference.
Don