You have less of a chance of any problems @ *90.
if another keyway is cut @ 180 then you have a chance of problems later on.
In an industrial situation, we had to look @ all angles so to speak,as
our machines/equipment ran 27-7, 365.
Downtime ran in the thousands of dollars a minute.
anytime you can prevent a problem or plan downtime, it was a plus.
A shaft cut @ *90 will not tend to become undersize/wallow like it will @ *180
Just like setscrews, they are set @*90.
