I have to agree with icculus308win and jaybuck43 it just is not worth the risk, at least that's how I feel. The price difference between a 9.6 and a 9.8 is quite abit that would feel scary losing. If there is even a slight chance for whatever reason the book comes back a 9.6 instead of a 9.8 (not saying it will or should) its just not worth the risk, and there is always a risk cracking a 9.8 and getting it signed. At the end of the day ask yourself if you would feel happy with a 9.6 SS, if the answer is yes then do it, but if you have to see that 9.8 grade don't risk it. I do like Jaybuck43's idea of getting a second print signed, heck you can even get a 9.8 SS on that, would still look awesome, not cheap of course but at least you wouldn't have to risk your 9.8 first print.
I just have a side question slightly related, I have seen this done on a couple tmnt 1 first print and other print slabs before, people would get Keven's and/or Peter's signature on the actual slab itself on one of the corners. This way there is no risk, you still get to see that blue label with the grade you have. Is this worth doing? worse case its just some marker on a outer plastic case and not on the actual book. I am debating on doing this or just leaving it as is, what are your thoughts? I probably wouldn't do this even on a 9.8 just to keep the case looking clean but for anything below...