The short version of my rant is that you should not worry about it. Pay headspacing on these kinds of guns no mind, other than replacing the bolt at the recommended intervals.
The long version of my rant:
Headspacing is not adjustable on these types of guns: the barrel, chamber, and barrel extension are (nearly?) always welded together at the factory, and the bolt lugs are pre-formed to fit a spec bolt in a very specific way. Thus, there's nothing you can do about it even if you wanted to.
If you are especially worried that some manufacturers may not have the correct tolerances, buy the bolt and barrel from the same manufacturer. For example, my mostly-DPMS M4gerie has a Bushmaster barrel... and a Bushmaster bolt. This gives me greater confidence that the headspacing is correct, even if Bushmaster had lousy tolerances (which they don't).
I wouldn't venture any further into it than that. Nonetheless, I'm sure the "debate" will rage on forever. I am awestruck by the number of people who worry about headspacing an AR when the gun is designed to make that a non-issue for the end user.