Here is the story I always tell when asked this question..
My daughter has a pool with flagstone coping. You can just look at the coping and identify the flaking stones. Some of the stones have very thin layers that just naturally flake off over time. The flagstone coping was put in about a year before we added the Salt Water system. The stones that were bad before we went with salt water, are still bad, but not any worse. The stones that looked good before the salt water system was installed, still look good, with no signs of deterioration.
I believe that if you could personally select the stones that were to be used on your pool, you'd never have a problem and would not have to seal anything. But... that is not likely to happen.
I'm also not sure how successful sealing would work on the bad or flaky stones. I can't see in my mind how sealing would stop the problem, as I think it is the make up of the stone that is the issue.
Jim R.