The two-grades-lower "thing" has always bugged me, albeit for a different reason.
Take your example of a PSA 9 OC. If graded a straight 7, some buyers would say, "...with that centering, I can only price it like a 5..." (looking to whack the card again when trying to buy it from you)
I wish
ALL graded cards had sub-grades. It would make things much simpler:
- Are you a "pointy corners" collector? Get the cards with a 9 or 10 for corners.
- Want to make sure there are no surface scratches that don't show up in photos? Subgrades should have you covered.
- Do you worship at the Altar of Almighty Centering? Go for it; the subgrade is on there if you don't trust your eyes.