I think the front triangle and the chainstays are the minimum sub-assembly that should be built at one time. I built a practice frame a couple of years ago with just the front triangle, and as a result the bb chainstay sockets are off by a couple of degrees. The seat tube and down tube don't really constrain the bb shell that much. With oval stays, it generally isn't as critical, but why not take advantage of the slop that is built into these parts?

the original Trek production jigs built the bike as sub-assemblies. You would put the top tube, head tube, and down tube together, and then put the rest of the front triangle together on another jig. The rear triangle was put on separately. They were somewhat problematic. I always liked the rear triangle jig though. The head tube was held in a piece of channel, which actually worked better than it sounds. The only issue was flux buildup. There was a lot of forced alignment, particularly on the small frames due to this design. The problem is that would throw out the rear alignment so it was a mess. It also tended to result in a top tube/down tube angle that was too small depending on the order of heating. Then putting the seat lug in the right place required the builder to put stress on the dt/ht/tt. I have seen other brands have strange failures at the head tube lugs that I suspect are a result of similar processes.

For lugged building in a vise with no jig, I like brazing the head tube to the down tube, the seat tube to the BB, and then completing the front triangle. Then the chain stays and finally the seat stays. My eyes are not good enough to get away with this anymore, and I wouldn't really suggest it to a rank beginner.