Hi! 

I realize you asked this question awhile ago, but I can provide some updates guidance now. I recently got back a plethora of PSA graded 1972 Sunoco NFL stamps - manufactured with serrations, to be pulled apart. They were all individually submitted either from freshly separated full mint 9-stamp panels (these were how they were submitted to gas stations), or from the large scale, poster Album insert panels. 

There was a key difference. Most 9-stamp panels were never serrated on every side. That is, if the stamps were attached to each other, they were serrated, but the sides of the stamps attached to the frame of the sheet were more of less missing serrations, so they have to be cut off the sheet itself. 

I submitted almost 100 individual player stamps over 4 seperate submissions. PSA returned some altered (trimming), and returned most graded. From the same batch they graded some 6s, 7s, 8s, a few 9s and even a 10. All from the same overall batch. Some were cut away from the sheet on 1 side, some more. The highest graded had no difference in their serrations from the lowest graded, or ungraded. Centering appeared to be the key. 

The inconsistency boggles the mind.

But I guess the one takeaway is that serrations, as long as they're intact, aren't held against the grades.