Puring does find the principle plane of oscillation (PPOP) and identifies the orientation at which the shaft will perform as if it were perfectly straight and perfectly round. We know that no shaft is perfectly straight and perfectly round, so the process helps eliminate an inconsistency that is inherent in golf shafts. SST PURE recommends the Pure mark go in the 12 o’clock position, placing the stiffest side of the shaft to the target. If yuo place the mark at 6 o’clock, it simply places the more flexible side to the target. It is still in it’s most stable plane. What difference can in make, or can player even tell the difference? It would depend on the player and how finite you want to get in analyzing ball flight data. In theory, if you want a slightly softer feel and want to enhance trajectory, go weak side towards the target. If you want a firmer feel and want to control trajectory and maximize distance, go with the strong side towards the target. Again, I doubt most would notice a difference and the important thing is that in either position dispersion should be improved.
As far as changing the lie by 6 degrees if you rotate it 180 degrees, I have no idea where the fitter got that information. I have no data or information that would support that. Rotating the shaft 180 should not effect the amount of droop (in my opinion, and experience), and even if it did for some players it would not be by 6 degrees.