I reckon that as long as a mod is easily reversed, it is fine. It's the idiots who screw extra posts in to the playfield and the various other botches we all no doubt see that need to be discouraged. Those are the 'modifications' are real hard to put right.
One mod I think all early Gottliebs and any other games with line voltage on the coin doors really do need is to rewire the start, coin & slam switches to 25v and add a couple of relays with contacts to carry the line voltage. Having a metal start switch and coin mech' switch only separated from, in the UK at least, a highly lethal 240 volts, by a thin and usually very old piece of fish paper is just madness. And none of the metal, the door, coin bar etc. is earthed. More madness. I, at least, add an extra insulator between the start switch and the contact, and earth the metal. A new fish paper in the coin switch is also a good idea.