I put an earth circuit on every game that I work on. I run the 3 core cable into the machine and put the live and neurals on and terminate the earth into a connector block, affixed to the motor board. From that, I run a separate earth up to the coin door frame and a link off that to the lock down bar. That makes it easy to disconnect should you need to take that board out at some point.

The replay button will get new fishpaper if necessary and I also cut down one of those black rubber tubes that are used on later machine playfield posts and that fits neatly over the end of the replay button end so theres no chance of any metal connection to the 120v line. I also put heat shrink tubing onto the flipper rods for good measure. I've had a few mains shocks over the years and it's not something you forget.