I'm in the "Do as you wish" camp but thought I would show my undercarriage. I wanted to go as original as possible and found when cleaning my undercarriage it didn't even get painted. You can see the nightmist blue over spray but no primer. So I tried to duplicate by painting it with a silver.
See the pictures, I have plenty more pictures also. The only red oxide I could see was a little starting in the rear and a little on the front rails. Not sure what happened in between.
You can also see how well the car has held up with no paint all these years. Check the frame rail galvanizing.