oooohhhhh, now I wish I hadn't asked...
EDIT-
i second that question.i read it several times to make sure i was reading it right
You know I was actually thinking about this earlier. If you tune it to be in exact tune in playing position, then it would be possible for the guitar to be slightly flat when vertical. This could be because of the slight pull of gravity on the strings and stuff. But, even if that were the case, I doubt that there would be much change in pitch at all, and if it was flat that probably can prove that hanging by the neck isn't bringing any problems with it.