Bill, it depends on the size of the sheets of wood. If I break a sheet down to the size I normally use for scrolling projects, then I lay them flat on a shelf. If it large pieces, I generally have them leaning up against the wall. If I lean them up against the wall, I try to do so without the sheet leaning a lot. I don't keep a lot of large pieces on hand, normally, as long as I have a piece that I can get a 24"x36" piece out of, then I'm happy. I do keep a fair amount of 1/8"bb plywood on hand, the pieces are 12"x24".
Len