Why not during boring? I would think that you would also want it when you bore, to keep the cut straight.
When you tighten the torque plate, it distorts the block, and you want that when boring as honing is not meant to remove any metal as much as just to "scuff the surface"....
If the block distorts when the torque plates are secured it may need to take off more metal than the hone is designed to do....