Thank you for those great citation and links!
I didn't knew Forrest Addy but as I've read some from him I found he's a great source of information. This sentence (taken from a post in this thread: https://bbs.homeshopmachinist.net/fo...-hand-scraping) describe perfectly the state of mind of scraping: "Scraping and stoning is a task involving all the senses and lends itself to a Zen-like contemplation."
LOL so true!

What he says about rotating randomly makes perfect sense to me. I mean, I am not a machinist, nor I have that experience and skills in this field, I come from electronic engineering and computer science after all. But for that very reason the randomization makes sense to me, because it spreads the distribution of the peaks making them to fall into the average.
That is what I've used to make my bars flat: I instinctively felt that despite rotating (flipping) the parts, it would have ended up in causing some sort of warp, so I thought that shifting the point of contact to "print" one surface against the other whould have randomized the result, or in other words made detectable the peaks and througs.
However after having read about the rotation by 90° and the square shaped plate conditio sine qua non, I was a little bit confused because my bars resulted flat nonetheless. So I've attempted to model a simulation to get an idea of what happens ...but the devil slips in the details and I messed up my simulation.
I think the final point can be summarized by the word detectability. If you can detect where the peaks are, then you're good to go.

Cheers, Cld.