Thanks BigInch, I have come across the second document. They are useful but still do not answer my question, as what I refer to is distortion due to solidification shrinkage during welding.
The 'distortion' mentioned in the second link (and also in B31.3, 335.1.1(a)) refers to deformation of the pipes to allow a good fit-up, hence a different kind of 'distortion'.
I go on "If I can see it is crooked, you have to re-do it" I am not going to have some kid 20 years from now bad mouth the design because the piping looks bad. I might not make friends with the welder, but I am not ashamed for other people to see the finished product either.