Sean,

In this case hanging the drywall vertical would result in fewer joints.
If the ceiling was 8'-2" or less, horizontal would be the way.

Sean wrote:
I have an older commercial building that I have to drywall and it has a
sloped ceiling. (It slopes about 1/4" every foot. It starts out close to 9'8
and finishes closer to 8'10". It's a very old building). I am putting in a
40' wall right down the center. Should I hang the drywall horizontally or
vertically?
Most people seem to recommend horizontal for most drywalling, but with a
slope like that it makes things a little more interesting.
Thanks