How critical is the stamped directional arrow on the power fan for installation purposes?

The stamped directional arrow on power fans is important.

It is always important to keep the arrow pointing directly at the peak of the roof. The collar attached to the base flange (the one with the arrow in it) has a seam in it that is oriented to be at the bottom of the flange. This helps keep runoff water from entering the vent. Depending on the overlap of the metal, the seam may catch water if it is not on the bottom.