Muzzle Brakes and Flash Suppressors do effect accuracy! They will also shift the point of impact for the entire group fired. Any thing attached to the muzzle effects accuracy and point of impact, whether it be a flash suppressor, muzzle brake, or sound suppressor.
This is due to the extra mass effects the barrels harmonics and all supersonic projectiles create a shock wave that radiates off the nose toward the rear. Any thing (muzzle brake, flash suppressor, and/or sound suppressor) that comes in close proximity to the supersonic projectile effects/deflects the shock wave against the body of the projectile.
Look at the effect as tested on a Valmet rifle using as mechanical rest.www.guns.connect.fi/rs/impact.html
No muzzle brake or flash suppressor with a target crown is the most accurate type of barrel, that is why varmint, sniper, and competition rifles don't typically have flash suppressors or muzzle brakes. The decrease in accuracy is typically not noticeable to most shooters. However the shift in the group is quite noticeable. Adding the flash suppressor shifted the group 65.3 mm (approx. 2.5 inches at 100yds).
For full auto, rapid fire, and/or military applications, the decrease in accuracy and shift in group is off set by the advantages of the flash suppressor or muzzle brake.