Thanks very much, Keith.
As to the tapered forearms, I don't know of any matching the butt angle -- thought I did, made a post over on Rimfire Accuracy, & Jerry Stiller corrected me. He did go into a bit -- at least the most recent incarnation. See post #20 for those tapered forearms:
http://www.rimfireaccuracy.com/Forums/showthread.php/2815-Sloping-butts?highlight=butt
As to the general phenomena of muzzle angle during recoil, there is another problem area: the front (at least) sandbag. It's been reported -- Don in Redondo being one of several, IIRC, that an edge of sand can form &/or break away inside the front bag during a record string. That means the height of the sand in the front bag may change, and *that* would give you the same problem.
I do remember Varmint Al saying the forearm can lift completely off the bag during recoil. And bless me, I think I remember him saying the butt does, too. Too many recent examples of a once dependable memory (never had to take notes in school) no longer so dependable. If I'm remembering correctly, there is an FEM of the load on the butt somewhere, and I believe it dropped to zero at one point.
All this points up the futility of trying to make an over-simplistic determination on what's "best."
Here's a story: I have a .338 I'm currently doting on. It does come back fairly snappishly. To counter that, I take a lot of the recoil with my forearm hand, pushing forward against the stop, and pulling down against the bag. Some very good groups with that technique. Then, at the July match, an 18-inch group. Not "vertical stringing" as we usually use that term, not exactly a 3 & 2, just ... depressing vertical.
(1) The rounds were prepared a month before, the batch had 6-inch groups. It was a proven load, and no single round made the group so large. Odds are very high it wasn't the ammunition
(2) No other competitor on my relay had that sort of vertical.
My current thinking is that after the first couple shots, I pulled down harder on the forearm & unloaded the rear bag. I'll have to test to be sure, of course, but it fits, and with your numbers, that explanation fits even better.
* * *
It's easy to get our heads oriented to where we think everything is in the design of the rifle, bench technique doesn't matter. Don't believe that's quite correct.
So for the original poster, I wouldn't worry about it, unless for not very good reasons you're convinced it is a problem. If you believe that, it likely will be. Otherwise, there are enough other things to worry about. Maybe after a number of years, butt angle could become a significant part of how you design your long-range competition rifles, but that would be based on how you've developed your 1K components & technique over time, not on an "inherent problem" with sloped butts.
FWIW