Like other posters have said many things can cause this problem, IE, the wrap, the Bumper, the facing at the joint, the pin being off center more than two thousands, or the insert or threads not being true or even improper finish.
If you are going to roll the cue, first roll it apart roll it toward a light source. Now due to the butt and shaft taper you may of may not see any light under the butt or shaft. If you see light it is OK, as far roll, look from front to back for any uneven movement. Now if you see none assemble the cue and repeat the procedure. If there is movement forward or backward of the cues joint the facing, pin, or insert is most likely the problem. However, it still could be the wrap or finish but to determine this you would need to place the shaft and butt between centers and spin it.
While spinning you would need to check it with a Dial Indicator to find the exact location of the problem.
I agree that a cue should not roll off, and I also understand that the only way most people can check the roll is by rolling it on a table. I personally insure that my cues will roll straight on a table before they leave my shop because I know this is how the average person checks them. I have found that many times all I need to do is repress the wrap or other minor things to correct a roll. For me this is to easy, and if it makes people happy and comfortable with my cues it is a small price to pay, because satisfaction and giving your customers what they want is what it is all about.
Just my thoughts!!!