I have to admit I'm a little surprised / impressed by the level of caution being expressed here. Good topic.
I'm an electrical engineer by education and too many years of hands-on experience, working in the field of electrical power quality. I have worked mainly / mostly with larger 3 phase systems (hospitals, industrial facilities), 480 VAC and do a lot of work with grounding and bonding of electrical systems, equipment, etc. Although the OSHA rules regarding protective equipment and Arc Flash have gotten a lot more stringent over the years, in the bad old days I've made more than my share of live measurements. I've gotten bit once - a 400Hz power converter at the factory; fortunately just hand to wrist, a light touch and not through the heart.
So I have to admit that pinball circuitry (6V / 30V, even the line voltage) never really concerned me - I had / could read the schematics, knew where the line voltage components and wires were so as to stay clear, knew to turn off the power working in those areas. I turn the power off to adjust contacts, replace components and such but mostly to avoid getting caught in a mechanically moving part. It still scares the crap out of me when I'm looking into an open / energized game and accidentally brush the flipper button. But yeah, a lot of time troubleshooting demands working live.
Of course, working with 40-50 year old games that many hands have worked on and with insulation breakdown, frayed wires, etc. anything could be live.
GFCI, definitely a great idea. Polarized and properly wired plugs, also important. If you are working on a machine, remove anything metal - bracelets, chains, rings, watches, fitness trackers that can become easy paths for electricity. And now that I'm thinking of it, I have a couple of wooden chopsticks in my pinball kit to use when I want to reach in and poke something or energize a relay during troubleshooting without getting my hand or a metal tool too deeply into the guts.