Generally you dont want to machine on a heat treated and stress releived barrel.
However, I have gotten barrels from Shilen and Lilja and have turned them down to the taper I wanted. The accuracy has not been affected. It depends greatly though on how much you take off and the imparted stress levels machined into the material when machined.
I took on the average .025 per pass.
If you have any doubts send it off to have it cryo treated and this should remove some of the surface stresses but not as good as heat treating would.
Be sure to use as small a radius (carbie inserted cutters) to avoid part felxing and chattering, if your using HSS make sure to hone your cutting edges and once again a small radius on the cutting tip would lessen part flex.
As always correct speeds and feeds will result in less stresses imparted.
IMO if you take light cuts but enough for correct tool pressure (when using carbide inserts) and correct speeds and feeds the stresses imparted will not affect the accuracy.
The Shilen barrel I turned produced groups of 3 inches at 400 yards with a 10 shot group and the Lilja barrel produced 1.7 groups at 400 yards 10 shot groups.
My personal Lothar Waltherd barreled 300 wsm
Win 70 produced extremely tight goups .2's and lower @ 100 yards, .2's @ 200 yards and .5 @ 300 yards consecutivly shot, all groups were 3 shots (didnt do enough loads up for 10 shot groups)
10 shot groups will show you if the barrel wants to walk or not.