Grain orientation in box spars is, and has been, a hot-button topic since Orville and Wilbur. Many years ago, the plans vendor for Piel aircraft designs in North America, Gene Littner (an aircraft engineer and designer specializing in wood airframes) maintained that spar caps need to be built from flat grain rather than vertical grain laminations. Here is an excerpt from his Emeraude newsletter . . .
This statement sparked a controversy which lasted for months, until Gene finally refused to discuss it further. I built mine with VG orientation because that's the way my rough spruce was sawn.