I received my new iMac two days ago. Last week I ordered 64 GB of RAM to replace the 8 GB that came with it.
Before adding the new RAM, I turned the iMac on to make sure it was working. Then I turned it off, unplugged it, opened the RAM door, and replaced the RAM without laying it on its screen. However, the back of my iMac faces the opening into the room it's in, so the back is easily accessible. I pushed the bottom of the iMac screen away from its base to give me the most room to access the RAM door. I think laying the iMac on its screen is the preferred way since it makes the RAM door easier to access, but I had no trouble doing it the way I did.