Yeah it's a tricky one isn't it. On the face of it, it sounds very plausible that there must surely be extremes in pellet quality throughout a whole tin and better prep is potentially a good idea. I don't think anyone on here can say with any certainty that pellet prep doesn't work. Although the anecdotal evidence to the contrary is pretty convincing.
It would be interesting for someone to do some downrange chrono testing on pellet prep. If the result was that there was a clear difference in downrange velocity and deviation then I think you could quite reasonably claim that it makes a difference. If it doesn't make a clear difference on downrange chrono tests then I think you'd be struggling against the weight of exterior ballistic theory to claim that it still makes a difference regardless of the chrono results.
At the moment, if you ask someone who's convinced that pellet prep makes a difference what evidence they've got to support their theory it's likely to be a claim that it made a difference to their group sizes. But, I think that evidence is too subjective and open to interpretation to lend too much weight to it personally. I'll be fair though, my opinion might just be down to the fact that I can't shoot groups consistently enough to be able to use it as a reliable test.
It's the same argument with things like barrel harmonics, twist rates, air strippers, adjusting air stripper cones to suit the pellet length etc etc.. the list is endless really
If someone was willing to do some downrange chrono testing on any of those things, I don't think there's a competition shooter on STB who wouldn't be interested in reading about it.