I nudge the heck out of my machines, and I've never hurt any of them. That being said, when I nudge them my goal is to simply "wobble" the machine slightly without the leveling legs sliding on the floor one tiny bit. If the leveling legs move on the floor, I feel I've gone too far. I don't see how you can hurt the machines by doing this kind of subtle nudging, unless you forget to bolt and clamp the backbox (and it falls onto the cabinet rails), or your legs are loose and you have no leg protectors and the loose legs start eating into your cabinet. I also leave a few inches between my backboxes, just in case, as I don't want the back box bolts eating into the adjacent machine's backbox should the nudging sway the backbox a bit. Now, if you have to have your machines really close in your line-up, I can see being concerned about the backboxes making contact with each other, and perhaps then I'd go really easy on the nudging or try to avoid it completely. I don't think I can avoid nudging to some degree, as it's an automatic behavior from 3 decades of playing pinball. It just feels natural to me.