It takes a huge amount of twist to get a "spring" effect--so much that if you get it, you definately need a new string that is the proper length.
Dacron stretches, period. That is one of the main reasons I prefer FF type materials, and don't like low strand count strings in any material. I don't like the stretch.
Flemish may stretch a tad more, but properly made the difference is minimal. I can stretch ANY string (endless or flemish, any material) on my stretching jig. Even stretching a string under approximately 300# of tension (which I do) won't fix everything, especially with dacron, because it retracts when the pressure is let off.
I'm switching over to B-55 because it has a little less stretch/creep, but it's still not in the same class as 8125 or Dynaflight '97.
How much stretch you get depends on a lot of variables: draw weight, draw length, strand count, temperature (you get considerably more in hot weather), how much you shoot, etc. etc. etc. As noted, you can up the strand count for less stretch. I'd leave my bow strung if possible for the least variation.
Chad