Hand tightening the cups won't tell you a thing. Some cups have thread locking compound applied to the threads at the factory, so hand tightening is no even possible. It takes some torque to force the cup against the face. If the threads are not aligned on both sides, that can't be fixed. Facing just makes the faces square to the threads, reducing the chance that a cup might come loose while in use.

There should never be any paint left on a face when a cup has to be tightend against it.