I do not believe double action makes the turn line any worse than the same number of cycles single action but I had not thought about it before. What double action can do is peen the edge of the cylinder stop notch. There must be a better word than peen but none come to mind.

I advocate stoning cylinder stop edges but only to retard the change in the revolver's appearance. Turn lines are normal but nearly avoidable. Think in terms of removing atoms from the edge, not whole molecules. You only need to dull the edges. Even if that can be spotted it certainly is less noticeable than an obvious turn line. I can not imagine anyone rebluing the cylinder stop after stoning it.

Do turn lines reduce value? With revolvers that are other wise 99% they absolutely do. Without stoning the cylinder stop the revolver will look like it has fired the number of round that it has fired. If its stop was stoned before a turn line developed that 99% gun will look like it was barely fired at all. In 90% guns it makes no difference.