I read somewhere that valve to piston contact tends to happen on high-mile motors because the stop in the head for the valve wears over time. It's a fairly tight clearance; not sure why it couldn't be more, but it is what it is. I wonder if it's the kind of thing you could catch in time - you'd think you'd see the flat of the valve that faces the piston start to scuff before anything major happens, assuming you checked it often enough. I suppose the best thing would be if you could somehow weld up the stop and then machine it back to spec, but that's way outside anything I, or most mechanics could do. I can't see any reason grinding a bit off the end of the valve would be a problem - good suggestion.