Yes, this sort of thing always turns into a crap-throwing contest where everyone loses. If someone says they align to .005" over the whole frame with lasers calibrated by NASA or some such, then everyone else is afraid to look bad by saying they only align to some lower standard and the people who built their first bike and just used a piece of string and their fingers start to think their bike that rides great has something wrong with it.
Forks aren’t straight. Wheels aren’t straight. Saddle rails/covers/padding aren’t even. Most bikes have a whole heavy drivetrain on one side and a much lighter set of brakes on the other so even if they were perfectly straight you’ve already built something unbalanced. Your feet/cleats don’t fit on exactly the same spots on the pedals.
When you go to face your BB shell and head tube, you inevitably have to reference the first facing cut off something (threads, unfaced end of the head tube) that also isn’t flat/straight.
And humans aren’t even symmetrical! I’ve got a bigger and longer right leg than left, and one foot that’s 1/2 size bigger than the other and I’m nothing unusual at all. I bet the right side of me is 100g or more heavier than the left side.
The reality is that I don’t think I’ve ever ridden a decent quality production or custom bike that didn’t turn how it was supposed to and ride fine with no hands. Heck, I’m not sure I’ve ridden a Walmart bike that didn’t do those things.
So the standard is “you pick how straight you want to try to get” but it’ll almost certainly be fine even if it’s your first try and the flattest thing you’ve got is a formica countertop. If you find it personally satisfying to pursue perfect straightness that’s great, but it’s not necessary. We built toys for adults, not centrifuges.
-Walt