Basically it seems like it depends how dark the ink spots are, and then, how extensive. If its pencil grey, and relatively minimal, it seems to be passing for 9.8's, if its darker, blackish colored, or spread throughout the back cover, they seem to be grading down.
Heres a good example of minimal ink stains on a raw that I think would be acceptable as a 9.8
They almost always appear by the top staple for some reason.
See how they're really light in color?
And this was the most "questionable" copy I submitted. This is the one copy zi'm not sure of, because the roller lines extend all the way across the cover, even though they're really light. I'm hoping it'll come back 9.8 with the rest of them, but we'll see.
I've scoured ebay over the past three months checking out every slabbed 9.8 with a picture of a back cover scan, so I feel like I have a pretty good idea of what passes. I havent seen any back cover scans of slabbed 9.6's and below, so ai can only imagine that its the darker, more pronounced ink stains that are bringing those grades down.