ASME B31.3 does not answer all questions about piping and piping systems. B31.3 refers to and endorses many ASTM standards
Basic permissible dimensional variations, including pipe length and straightness requirements, are defined in ASTM A530, this standard being referenced by all ASTM pipe manufacturing standards.
I think OP is talking about straightness after fabrication. Refer B31.3 requirement below which refers to miter bends and indicates the max angle for consideration as bend.
So max angle without design consideration as bend is 3 deg. But you need to consider other aspects like drainage slope and loss of supporting effort from bottom supports which may need to be adjusted.
Engineers, think what we have done to the environment !
