I had same problem when I made spells and calculated a lightning to be created from point X to target point of ability being cast.... and found out that the caster doesn't always turn to the expected angle.
Sadly, I don't really know how to solve this completely - if there is any custom script that forces the unit to face the exact angle, then it's unknown to me.
I solved this problem by ordering the unit periodically to face the angle (like every 0.06 seconds to face angle X over 0.00 seconds). That seemed to force the unit to face the correct angle over time. But it ain't a really nice solution imo.