Most of the studies available online say it's not worth the cost due to minimal (if any) difference discernible by the golfer using them.
It is an interesting discussion for sure. As someone who built clubs for many years, I always FLOed my shafts. It was not unusual in older graphite shafts to see 1/2 flex or even full flex differences depending on which way you aligned the shaft. The last few years I built, the quality improvements were noticeable and the "bad" shafts were fewer. If I were still building, I would still do it as it does not take long but I'm not sure how much of a playability difference there is on today's shafts.