I always ground the transformer frame, then run a green 16awg wire up to the coin door frame and then a shorter wire from the frame to the door. On early gottliebs also to the siderails because the fipper actuator wires go to a metal pushbutton.

Always check the fish-tape insulation is in good shape on the 60's gottlieb start pushbutton contact. That has shocked me with 110Vac many times until I got smart and grounded everything and made sure that was still there and not bent to the side. I don't know if the Bally's are the same but it wouldn't hurt to ground as much as you can especially if kids are playing the machine or it is adjacent to other games you can touch at the same time accidentally.