Yeah just in case there were actually someone out there who might just possibly care, I decided to post this up.
I just figured out something small tonight. I used to talk to two different old-timers who round filed chisel chain, and they both told me the same thing though I could never figure out why. They said, "Always file up at a ten degree angle." "Why?" I would ask, "Since the manuals all say to just go across at a straight 90 degrees to the bar." Robert: "Jed... (eyes rolling) just do it. I've been doing this a really long time. Do you think I might know what I'm doin'?" Max: "Just the way I was taught to do it, and it's always worked out real good."
So anyway, I think that tonight, I just figured out why. When you sharpen without raming a wedge in the bottom of the bar, the drivers naturally rock over in the bar grove from the pressure of the pushing file(on a moderately worn-out guide bar) about ten degrees. One compensates for this with the ten degrees in the file. If you don't, the top-plate comes out with a more acute angle that the twenty-five or thirty degrees that you intended.
...... For those who might possibly care.