Is there a good reason character models should be rotated -90?

I’ve been using Unreal since UE3/UDK and we were always told to set up the character so it faces down y forward in the modeling program, then export it and rotate it 90 degrees in game to face down X.

Why can’t it just face the correct direction in the modeling program?

This creates some headaches when I do IK related things with my character and it always ends up being rotated 90 degrees and I have account for this extra transform all the time.