The depth of the threaded portion of the Ops-Inc 12th Model muzzle device is set up to be used with a crush washer. A stack of shims thick enough to properly time the muzzle device wont be as thick as a crush washer, and the threaded portion of the barrel will protrude slightly into the first chamber of the muzzle device.
Will the sky fall? No. But, it's just not the way things should be. The gun will run fine without a suppressor. But, it will screw up the proper spacing between the collar and the muzzle brake for mounting the suppressor. If you're actually going to use a 12th Model suppressor, the brake will end up too close to the collar and you won't get sufficient turns of thread engagement.
In the early days of the 12th Model, shims were used before crush washers were. And the depth of the internal threaded portion of the brake was commensurately longer.
I only use an adjustable wrench on the web of the 12th Model brake for the final quarter turn of tightening with the crush washer. I'll use an old A2 flash hider and a 3/4" wrench to get the crush washer very close to where I want it, first.