Gotta be honest... I'm firmly in the "don't touch my stuff" camp.
That said; I have been known to be so exhausted or constrained for time after setting up my machines at TPF (I bring them all - at least 5 machines) ... that I simply forget to check for levelness. Usually; I do play at least one game after setting them up and if there is a gross level issue I fix it.
That said; I would really be ok with an attendee taking matters into their own hands - so to speak.
As long as the Attendee is willing to take the risk of liability. That is if they damage the machine; they have some 'splainin' to do.
That said; its just wiser to leave me a post-it-note... or tap me on the shoulder.

That said; Turning off a malfunctioning game is the only "real" way to let the owner know it's broken beyond use. IE a stuck ball which prevent continuing game play... or non-functioning flippers. I check on my machines regularly; but do not play them. An "off" machine is the only way I know for sure there is an issue.

Because - It's already a chore to shop/wax/test/repair, breakdown, drive w/ trailer (hours), and then set back up. Further; you know you're going to have to work on the machine (or at least I do)… so by the time I'm done setting up 5-7machines; I'm exhausted... hungry and thirsty. By the time I'm back from solving the last two; the show is open and I can't work around the attendees.

No; my machines are never for sale. Always there, working for free play only.

Thank you for leveling machines for other attendees. just be careful with my stuff.

Not a chance in hell that will ever happen now... unless it's here on the website in the promo boxes.

Aint that the truth.
I once took my brand new ST:MU homebrew to a local weekend maker event here in Austin.
I had to walk away because of all the kids double flipping with no real understanding of what they were trying to do. On one hand; I was happy they were experiencing Pinball for probably the first time... on the other I was disappointed they were "Free range" without their parents really supervising anything. It was clear that a lot of people didn't understand what Pinball was.