I've been collecting American coins for years, but nothing of any great value. I'm really just interested in the artwork on the coins. My father collected coins and currency from around the world for decades and I inhereted that. I've added very little to it, but that's mostly because I don't have the sources he had. He worked for the State Department and would buy money from peopel who had just returned form this country or that. It isn't worth a whole lot. After all, he was a government worker with four kids to clothe and feed, but it's really an interesting collection. If there is life after death I'll just have to tell him about the Zimbabwean money I got. I'm still looking for a 100 trillion dollar note. He'd be pretty tickled.
I would never polish a coin. I am not a serious collector but I have some coins I have aquired over the years and original is always what any collector looks for. Btw, we could have a historic top in the gold market,along with stocks. The old adage,when everbody wants it,give it to them,might apply here.
One simple rule currently is to retain any dimes or quarters from 1964 and before. Their silver content makes them worth many times their face value, with silver being ~ $24/oz. right now. This is not really numismatism, but sheer economics driven by the falling dollar and market positions.