Without a chamfer, I find a small pile of copper shavings on my 650 shell plate after a few hundred rounds or so. Drives me crazy.
I see so you have observed that a square edge will or can shave some copper? My final step is to drive the expander ball down the case which leaves a clean square edge. I can see if the seating die isn't square it can cause an issue. Before I bought the Dillon, I had the manual hornady trimmer and that left a heck of a burr that I would take off with a chamfer bit. I have to watch my 650 closely next time I reload .223. I reload .308 on a single stage press and haven't observed any shaving even with a flat bottom projectile.
I read the Giraud website and they mention their chamfer is especially important for boat tail projectiles. Maybe the boat tail projectile is more prone to shaving?
Chris