I repair most of the larger machines, I have seen many minor modifications, power cords, jacks for coolers etc. my position is if it didn’t cause or aggravate the failure I don’t worry about it.
Now if you’re in there modifying the way it works, IE: installing some third party plasma CNC interface and it causes a short and catches fire, you’re on your own.

So yea, it's on a case by case basis, I think we are more than fair, just use your head. If you fry the solenoid relay for instance the biggest issue for us is liable to be over the shipping.
I been known to do some McGivering myself, but you have to take responsibility if you Eff it up.