if you think back to the days of live axles the angle of the prop shaft changed continuously as the diff was attached to the rear springs. If you loaded the boot up the angle could change by several degrees - more when you went over a bump. The moving joints in the propshaft are there for that reason - to allow for misalignment. You want to get it as close as possible to give the maximum movement in all directions but it does not have to be perfect.
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