The root rib bearing spinning during assembly is something all Grobs have in common. With the ship assembled, I would check for any vertical movement at wing tip (with the other wing tip on a stand). Pick up wing tip with about 25 pound force and check for slop. You can feel any movement. If the wings are tight, I wouldn't change the bearings. You could consider gooping up the bearing to keep it in proper alignment for next assembly, something that would keep the bearing from spinning and still allow slight movement. The heavy wax used in a toilet ring might do the job.
Hope this helps,
JJ