Fittings are designed with the angle built in.
It's never an issue.
If the drain line is sloped 1/4" per foot, then wouldn't that force the trap riser to be tilted from vertical, and therefore cause the clamping drain to be out of level with the floor? There must be a simple solution...what am I missing? Are traps designed to slightly more than 90 degrees to compensate? If not, then how do you achieve a 1/4" per foot drain line slope and also have a clamping drain that is level with the floor?
Is the trap designed with a built-in correction?
It would only be a problem with screwed connections and they do have the pitch taken care of. PVC and ABS joints are "flexible" when first assembled, so there is a fair amount of "give" to make the riser vertical, with almost ANY reasonable slope, not just 1/4" per foor.
Licensed residential and commercial plumber