I was worried that I might have done damage to it, so I called Apple again and spoke with a different customer service rep. So, after I cleaned my iMac it seemed like the screen was more difficult to read and see details, and for the first time I actually noticed individual pixels without looking for them. The woman on the customer support line said I could use rubbing alcohol to clean off the screen, which didn't seem to be that unusual to me since I've heard of people cleaning electronics with it before. Prior to doing any of this I had called Apple to see what kinds of cleaners I could use on my iMac. I used the rubbing alcohol straight out of the bottle onto the paper towel - did not dilute it. My iMac had a very visible film of rubbing alcohol residue all over it after I finished wiping it down with the rubbing alcohol, which I cleaned up with a dry cloth. The paper towel wasn't wet enough to drip, but very close to it. It took 3-4 tries with a paper towel soaked in the rubbing alcohol to get it squeaky clean again (I have a skin condition that requires me to put lotion on my hands/forearms and over time the lotion managed to get all over the screen and around the edges of the iMac from moving it around).
So I cleaned the outside of my Late 2013 21.5' iMac with rubbing alcohol (70% isopropyl alcohol), including the screen and around the edges of the screen.
