Cues in wall racks, or even floor racks, for that matter, generally do not fair well when fully assembled. The reason is, the cue still depends on the top portion of the rack to lean on...over time, it can cause warping, particularly in the shaft. Look at all those barsticks that have wobbles in the shaft area from wall rack storage.
The reason for this, and another reason NOT to store a cue, even disassembled, flat, is because gravity works! The force of gravity pulls on the one side of the shaft that is leaning against the wall rack, or stored flat, pulls down on one side of the butt and shaft. Over time, this can cause warpage.
Cues should always be stored, disassembled, in an upright position...whether that be in a display case or in a quality cue case, in a humidity controlled room.
Lisa