I agree with any super accurate method of aligning that professional builders use, but the average home builder can obtain excellent results with a straight edge. I too have seen several misaligned and that was due to not being aligned at the onset (even with a straight edge) or had terribly damaged shrouds. 38 builds with a straight edge and so far I'm not aware of a problem. In addition to the straight edge method it's real apparent if it's not aligned because the backplate doesn't engage uniformly in both side plates and the internals don't function smoothly...so far the straight edge on our builds has produced neither problem (that I'm aware of).
Also, I cannot agree with "all barrel shrouds are bent". I have several and they are plus or minue a few thousandths from end to end. I've been using shrouds from Angola Armory, Sarco and ORF. Maybe the parts sets available now are less quality and builders should defnitely be checking their shrouds if that's the case. One point to make here, if "irregularity" and not "bent" is meant then that I think is correct. The tutorial says to measure at the trunion end and the muzzle end and says nothing about the middle because most barrel shrouds have irregularities including highs and lows in the middle, but has nothing to do with front and rear alignment. The gap should be measured only at the ends and not in the middle.
If anyone has carefully followed the barrel alignment tutorial and had a problem with an out of square receiver we'd like to know so post it here. For many years using the straight edge or laying the weapon on a flat surface has been used satisfactorily by home builders that do not have the tooling to do otherwise. I use a straight edge because all my benches are made of wood and I don't have a "true" bench and a straight edge is nothing more than a "true" bench turned upside down.
I also used a straight edge to check the alignment on my OOW built '28 water cooled by laying the straight edge on the water jacket and measuring down to the receiver at front and rear and it measures dead nuts on so I know the straight edge works for us less than sophisticated builders.
I can't speak for other builders results, but the real proof is on the firing range and on my personal builds I've set the rear windage sight at dead center on the bench and it might require a one or two click adjustment at 100 yds...close enough for this home builder

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The gist of my comment here is that the home builder need not fear about getting their receiver square without the use of special tooling or having your weapon professionally built. Just my .02.
I agree, galling on the internals is a sure sign of misalignment. Here's a pic of my 3 year old bolt with 1000's of rounds fired...parkerizing on the rub areas is still over 95% unmarred.