Wall racks in most cases do not hold the cues perfectly vertical. Since this is the case unless modified, cues in these racks will lean one way or the other. Now since the shaft has the smallest diameter of the entire cue, if it is resting to one side or the other the weight of the cue focused at that point. To prevent warp-age from this, first make certain that base of the rack and the top of the rack are as level as possible, by using a bubble type level on the top and bottom of the rack. Then I would recommend placing inserts in the top holes of the wall rack that will keep the cues as straight up and down as possible. Inserts can be easily made by cutting and gluing in circular soft foam pads, make sure that the foam you use is durable enough to hold up for this use. Once the pads are glued into the holes find the exact center and punch a hole for the cue tip and shaft to slide through. This will keep the cue vertical in your wall rack all of the time and gravity will have little if any effect.
Last but not least, make sure that your wall rack is mounted in a temperature controlled room and that your wall rack is not located in direct sun light from a window. Believe it of not the Sun alone constantly beating down through a window will also effect the stability and color of materials used to build cues. Now many high end cue makers today are using UV automotive finishes on their cues, it is still best to not leave your cues in direct Sun Light to long periods of time.
Hope this helps