I tend to downgrade my raw books when i am evaluating them by eye.......i.e. if i'm fairly certain a book is a 9.8, i will look at it as a 9.6 or 9.4. I mean, you can thoroughly inspect the book inside and out, but there could always be a small little smudge inside the book that you might miss, which will bring the book value down.....and it doesn't take much.
Also, when i am evaluating my books for value, i will cross-reference 3 main variations......i use Key Comic Collector, GoCollect and Ebay Sold Listings to figure out a price.