You only have to paint it if you want it to look pretty. Most of the components that are bolted to the back panel are isolated from it, the ones that you have to worry about are open circuit boards(ie, the adjustable power supplies he uses) with solder on them...if you encase these, or mount them on some sort of electrically isolating pad, you'll be fine. Also, the holes through the plate in contact with metal mouting bosses/screws make this a moot point. That said. If you are going through all the trouble to make thigns pretty, painting will make things that much prettier.
