I shoot 1000yrd comp too, I de-burr most case brands like Rem, Win and Hornady or other US made brass. In fact, my lot of Winchester brass is just as accurate as my Lapua brass, once prepped.
The Lapua and Norma brass I use I don't touch the flash holes. I once had new Norma brass and due to not using it before, I decided to de-burr before looking into it further.
I felt quite some resistance to the drill entering the flash hole, didn't think much of it at the time. That brass shot terribly in testing, after talking with a fellow member at the club, he advised me to NOT touch ANY European made brass, it is DRILLED and is .078" flash hole, not .081" like US brass. The untouched brass is AWESOME in uniformity on case volume and the flash holes don't need cleaning up.
My Lapua brass is by far the best brass I have ever used, wish I had of kept more of it, didn't know at the time that Lapua would stop doing 300WM brass, the 300 I have will have to last, I guess. Haven't lost a single piece to a failure yet.
I feel the brand of brass is where it needs to be decided if the flash holes NEED to be prepped. I have some very clean flash holes on Winchester brass, and some that were not so clean.
Cheers.
gun)