Winter is coming; should the A/C unit outside the house be covered in the off season ( my neighbor says yes ) or left uncovered ( my gut feel )? Thanks

It is a toss up. By allowing the unit to be uncovered you can allow leaves and other debris to find there way into the unit etc. If you cover the unit completely you can trap moisture inside the unit and that can cause rust and corrosion. It can also cause a problem if you forget to uncover the outside A/C unit when you start using it in the Spring as some people do forget this. My solution has always been to just cover the top of the A/C unit. This prevents dirt and debris from entering to unit but allows the air to circulate thus preventing the rust and corrosion. In my case a spare plastic garbage can lid along with a heavy brick to keep it in place works well. Just remember to remove the brick and the lid before using the unit.

NOTE all bets are off if you have a heat pump since we all know you cannot cover a heat pump in the winter if you plan on using it.