I have never covered one and have never had any issues. We do give them a Lite rinse though just to be on the safe side. Then again, we rinse everything. Kind of tough to cover them anyway when they keep turning on and the fan kicks on.
We cover them, not only for SH caustic effects on the coil, but to also to protect the skin, takes two minutes. I think if a HO calls me with rusted screws someday, I can say with a pic that we did cover and protect. Necessary, probably not. We did discolor the skin with gutter wash once, too strong, not enough rinse, or no rinse.
Yes I cover, for the few minutes I am in that area. I just pull the disconnect if the customer is not home or have the customer turn it off for a few minutes if they are home.
We ask the customer to shut down the a/c, while we are there. I use a piece of plywood and place it over the top. Like others here we rinse during and after our roof treatment. Being prudent is in our best interest.
Serving the Pocono and Lehigh Valley(Carbon, Lehigh, Northampton, Monroe, Pike and Wayne Counties), PA community as well as both Warren and Sussex Counties of New Jersey