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I have a large 4-jaw chuck, of course, and yes, it is the only choice where the best accuracy possible is needed. The 3-jaw is just more convenient for most jobs, particularly now that it's a lot more accurate than it was. A 3-jaw, however, will never be as accurate as a 4-jaw.
As for grinding the jaws, I made a holder to clamp a Dremel in the toolpost and made a ring to fit over the outside part of the jaws so that the jaws could be tensioned against it. This was the method recommended on a couple of youtube vids. The chuck was rotated at about 100 rpm and a suitable grinding wheel fitted to the Dremel (rotating opposite to the chuck) and slowly wound in and out so that it only just touched the jaws. Coating the jaws with a black marker helps to see what is being ground off. I found that tensioning the jaws with the ring, however, gave inconsistent results as it caused the grind to be at a slight angle, front to back.
As my chuck has very little wear in the scroll I tried something different, I cranked up the chuck speed to near maximum and used centrifugal force to exert pressure on the jaws, then slowly fed the grinding wheel in and out. This actually worked.
I've bought two VFD's recently, a 2HP from a Victorian company called Conon (about $200) for my belt grinder, and a cheaper 1HP from an ebay seller ($104) for the lathe. The more expensive one has a cooling fan, the cheaper one has a large heatsink instead. Both work fine.
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