Simeon,

A more complicated answer is that vertical grain will generally be slightly stiffer than flat grain because of the role of shear modulus in beam deflection. How much stiffer can depend on the species and the particular sample of wood. For example, based on the Wood Handbook data, the difference between the two orientations for western red cedar should be insignificant (http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr190/chapter_05.pdf).

Sam