Interesting thread. I googled RC sheer webbing pics out of curiosity, and apparently it is not *that uncommon for ARFs to come with the grain running parallel to the spars.
I had to think for a minute as neither scenario seems ideal to me. I would think diagonal balsa square stock would be stronger and lighter, but then I also get that from a manufacturing standpoint, the perpendicular-grain sheeting would be more cost effective (labor) and still work for normal operation.