The flutes run width wise. I have not yet used the corrugated stack for 8x10s. Most of my 8x10s are on RC unless I'm experimenting with new paper types. I think I would be inclined to only do two 8x10s per layer since I don't have a lot of over-hang with the cardboard.
This weekend I watched a documentary about Henri Cartier-Bresson and noticed, as he was thumbing through a bunch of prints (11x14, I think), they looked wavy along the edges.