Building a frame, there are different ways of doing it - some people put the couplings in a plain tube first, others (like me) build the frame, then cut it in half to fit the couplings. I suppose you could end up chasing your tail, but really only if your temperature control is way off or you're being obsessive.

The offset is almost always vertical - if it wasn't, there would be a torque on the frame putting the HT and ST out of alignment. The offset, if there is any, is usually a small pull inwards - the two HT joints tighten the front triangle up a bit. If it's only a few mm I just leave it, if it's more I do a gentle cold-set, really to make it easier for the owner to assemble and dismantle the frame. With my own frames, there's not usually any offset.