Once on Jim's heads and once on a set of my own, and once on a medium riser set. All the ones I have seen are on pretty serious solid roller applications, although I have never gone to that 400lb spring level on anything here - probably 100 lighter than that at the most.
When I saw it on mine I assumed it was because I had radically milled the intake face when I made other changes. I think its a twofold issue - the head bolt insert area give the crack a "place to start", and the rocker load gives it a "reason to happen".
I have not done it - but suspect that we could fabricate a mounting plate that would go to the root of the rocker boss holes and leave the rest of the head unmodified.