I guess we've had enough replies now to get a reasonable picture.
Hard to draw any cast-iron conclusions but a couple of things spring to my mind.
1. I didn't see much evidence for the "748's are more prone cos they're revvier" argument.
2. It would seem that extra-frequent oil changes, and, indeed, the use of 15/50 synthetic, whilst in themselves no doubt "good things", do *not*guarantee to prevent rocker failure.
3. Likewise with extra-careful warming-up procedures.
Maybe how you ride, even with a hot engine, has got something to do with it. Spend a lot of time around "the red zone"?
I reckon variable batch quality in the rockers looks like the biggest single factor.
What do youse guys think?