Doh!!! We do our sheeting horizontal in WA too. You want to hit the most studs with each sheet. Same on the roof.
Roof is one thing, walls are another. How many nails go into 1 horizontal sheet w/o blocking?
Here, we nail each sheet 3" OC on all edges and 6" OC in the field. I've seen it called out to use #10 nails 2" OC before. It's hard to believe that doesn't actually weaken the edge. But the City of Los Angeles Dept of Engineering Testing Laboratory is one of the most respected authorities in the field of testing building procedures and products. In fact, a lot of building materials carry 2 listed ID's; one from the ICC and usually one from the Los Angeles Testing Lab. They certainly don't get used w/o the LA ID around here.
The point is, we sheath walls for earthquakes. If your situation does not require that much strength, by all means nail the stuff up anyway you like. It's only 'backing' when it doesn't conform to shear strength.
Oh, and BTW, I've had plans that showed plenty of horizontal shear too. That means full blocking in the roof plane. But since the load is completely different, panels laid up and down are weak on roof structures. So they do always go the long way.
Don't make walls and roofs the same, they're not.