From what I have been told, cutting from both ends does not affect the spine of the arrow. However, it can actually affect the overall straightness of the arrow. The way I understand it, the "straightest" part of the arrow tends to be the center. The farther out from center you move (in either direction) the "less straight" the arrow is. That being the case, you could theoretically take an arrow with a straightness of say .006, trim from both ends, and end up with an arrow that is .003 (this is just a number I pulled out of the air). All this being said (and assuming it is truth), I don't think there's too many, if any traditional shooters out there that could consistently tell any difference in point of impact between a .003 and a .006 arrow.