I think it depends on wether the mark is a misprinted mark like a fisheye or ink smear or something like that, or if it's something that's on the plate like a scratch or a section that wasn't made quite right.
The misprints should be treated like any other misprinting mark.
The plate problems should be treated as just the way all cards like that wee made.
Other stuff that goes wrong with the printing is a bit more of a gray area. One T206 I sent in got a 40 probably because of a paper inclusion. A bit of foreign matter, in this case a small hard object of some sort that is actually inside the cardboard. You can see it as a white dot on the left just below the underarm area.
So, it's as manufactured except for a tiny spot where the ink over the paper inclusion area wore off. Technically a scuff, and they could have treated it more harshly. Or they could have treated it as a natural effect of the printing process and given it a higher grade. Even under a magnifier the corners are as nice as they look.
Steve B