The best way to straighten it, and keep it straight as possible, is to hang it up by the tip end, when not in use.
I gotta say, I don't know too many that go home after league and hang their cue up....now I need to build a closet just for my cue storage.
97% of shafts out there have a WOBBLE, and I would go out on a limb to say it wasn't due to not hanging the cue up. Most cuemakers store their shafts while making them, in a cart just standing on end in their shop.
Also, if it was just done by a cuemaker, take it back, give them a chance to make it good, personally, a slight wobble in the middle won't ever be a reason to miss a shot, JMO.
I'm not trying to be condesending, and I hope it doesn't come out this way, but...
The Taper Roll?, IMO, there is no such thing as taper roll. If anyone believes it does exist, then explain what it really is, and how it happens? Any other material in the world that you machine that same taper into, doesn't do that, only a wood shaft, so if anything, it should be called maple roll or ash roll or purpleheart roll, etc. right? The Taper has nothing to do with it, it's just a term some cuemakers/sellers have come up with to sell a cue with shaft that has a slight warp in the middle. Simple as that.
People need to understand that wood can be a sponge for moisture, and it can have internal stresses and will move either slightly or in a big way depending how it's treated. It is possible for a shaft to warp once you take a bit off, especially if it was sealed in a set condition then recut, thereby opening the wood back up to the world and the conditions are now different. Winter to summer/summer to winter, dry to humid / humid to dry. I don't know who in this thread has ever built a cue or shaft, but I have some that are in my shop that move all the time. A couple are straight in the winter, but are warped in the summer humidity, then I have some that are totally opposite. I keep them around just to be able to really tell when the humidity changes.
I would bring the cue back and see what the maker will do to correct the wobble
Just my ramblings,
Good Luck,
Dave
Part-time hobbiest cue-maker