At Grand-Olive Billiards in St. Louis, regular customers stored their cues in what I would call map cabinets, that is cabinets with a lot of shallow but long and wide drawers, say 36 inches by 36 inches by 4 inches.. Many of these cues were two piece cues made out of one-piece full splice house cues. St. Louis is very humid and Grand-Olive was only a few miles from the Mississippi River. No one that I knew who stored their cues at G-O in this manner ever had a warping complaint, and many of these folks were chronic complainers. So my conclusion is that horizontal storage is ok.