The case halfs straighten out when you tighten them together, don't they?
Seriously....I never checked that part for trueness. I just always put the parts in the lower case, and slowly tightened the screws in a crossing pattern. If both of them are bent, as long as they flush up.....the bearings are tight on installation. If they don't fit in with an interference fit, you will destroy the gears when they walk around on you.
I will check them again on the next one I build.