I've known several collectors who clean coins with ketchup. For pennies, many use a pencil eraser. But you have to know your coins in order to know which to clean, and which to leave alone.
Wow. No.
I've been collecting coins since I was a tot and one of my grandmas gave me a small set of early twentieth century coins. Later she gave me souvenir coins from her world travels, and one of my grandpas, a doctor, gave me lots of souvenir reproductions handed put by drug companies as part of their advertising campaigns ("In colonial times when this penny was used, doctors had to rely on..." yadda yadda. There seemed to be hundreds of them in the 1960s and 1970s).
None of them, of course, were terribly valuable, but even then I knew the golden rule of coin collecting: don't clean the things.
Not with ketchup, not with an eraser, not with soap, and not with silver polish.
When I grew up my interest was in older coins. Perfect circles of identically stamped metal with minute differences making for fabulous differences in value did not interest me. But ancient coins did. There the rules are a little different, since many of them come from buried hoards and cannot help but have been cleaned at some point.
But even there, even though it's more okay to handle them than modern coins, you don't clean them.
You just don't.
By the way, one thing I have learned is that coins are generally worth less than people expect or hope. Roman bronze coins, which are what really got me started, have been dug up by the millions in Europe. When I started out you could get a coin in pure bronze the size of a nickel with an identifiable emperor on one side and something actually interesting on the other for $10 -- a price which has held steady for at least twenty years.
I don't know much about the values of modern coins, but I have hung around coin shops enough to know that most of them are ethical and fair and more than willing to help people learn about what they have.