I have a nearly unlimited amount of cedars at my property and also help on the Boy Scout fundraiser where they pick up Christmas trees after the holidays. I am definitely not wise enough to answer the "standing or lying down" question ahead of time, so I would recommend learning the best practice by experimentation. Especially if you have plenty of cedars available to you.
I would cable a few together with a cinder block and try to have them lie flat after they get fully water logged. If they are roughly 4' wide, then that would provide cover to within 8' of the surface. I would also try for a larger pile or a standing pile where you get the top of the pile within 4-5' of the surface.
Even if your crappie never thrive, I don't think this will be "wasted" labor. We would always fish the deep brushpiles for bass during the hot part of summer with a Texas-rig plastic worm so you could work back into the heavy cover. Some years, that would be our best dog days of summer bass fishing.
