Altering a coin "because it is warranted" (especially to just a few spots) is just plain crooked! That is not my opinion.
Since I consider "patina" is green and reserved only for ancients I call it recoloring them. However, I will agree that turning a pink large cent into one with attractive brown patina is also criminal. It is also an art, I am guilty, but if you read my post I never claimed that I WAS NOT ALSO CROOKED did I?
So, how much rotten hamburger can you mix in with the good stuff at your restaurant, and how much can you water down the AIDS or cancer medicine? A FAKE PATINA IS NEVER WARRANTED!!!
It is different for ancients than modern coins. Ancients are cleaned and sometimes need repatination in spots if the scalpel slips or something similar. Modern coins are a totally different ball game. Obviously, applying a fake patina is not ideal.
I don't actually do this, but in certain, limited cases- like accidental damage through cleaning- I think it is mostly OK and wouldn't expect it to be disclosed by the seller.