Correct. 4 companies produce 95%+ of all forged recievers.
http://www.ar15.com/forums/topic.html?b=3&f=12&t=318113
As long as you avoid Olympic, Heese/Vulcan, and other cast ones, you'll be fine.
Recently bought a bunch of Stag lowers. Just as good as my RRA, Bushmaster, Colt, etc...