If it's something like a Lemond Poprad with oversized tubes, it's probably best to ignore it if the front triangle is a little out. There are definitely pictures out there of buckled oversized downtubes.
A bb post isn't a good place to reference off of for just measurement. It's really unlikely to survive the joining process in a flat cylindrical state. There is a Paul Brodie video where he talks about that, I have to say that even after almost 50 years of building I learned something from that video. Vintage bikes are unlikely to have had a post-construction bb facing. I know we didn't do it at trek when I was there. We did lots of things a little off, tbh.
OTOH, a bb post is a good point to bend things from, so it's useful to have one. The other thing to consider is if a vintage bike made it this long without being aligned, is there really any reason to align it if it's not displaying serious handling problems?