i had a pro retaper a shaft and it ended up like a walking stick. i had southwest retaper a shaft. they said it was perfectly straight and would thus cause no problems in the retapering............they screwed it up.
now,,,i only trust myself. depending on how much you want to take off,,,start with 220 grit,,,then 400, then 600, then 800,,,1000, 1500,,, 2400,,,3600,,,5200,,,6000,,,8000.
sand with your hand wrapped around the shaft and ALWAYS rotate while you are sanding. apply an even pressure, NOT A HARD ONE, because you cannot maintain a hard pressure, and this will cause unevenness. sand the shaft in at least 3 or 4 different ways and always count the number of strokes you apply to a given sanding. this will insure an even sanding that is not biased or "one sided". make sure you sand through the tip to avoid that concave curve that most people get when they stop at the ferrule...the REAL problem most people have is this last. they sand the shaft good and perfectly round, but they stop at the ferrule, so they get that familiar concave bow just in front of the ferrule.
practice on a throwaway shaft/cue.