Dynaflight '97 will stretch, a little at least--I've yet to find a material I couldn't get a little stretch from on my jig. Obviously the fewer the strands, heavier the draw weight, higher the temps, etc. you'll get more.
Just to clarify, "stretch" is elasticity--it will recover when the tension is removed. "Creep" is permenant elongation--it doesn't recover when the tension is removed.
Every string material I've ever delt with has some of both to some degree. Draw weight, strand count, type material, etc. all play a part.
I can stretch an endless string with zero twists in it, but the twist of a flemish does have to settle in.
Contrary to misinformation I hear now and then, the twist in a flemish string does not act like a spring--settling that in would be considered creep, not stretch, as it doesn't recover (like a spring does).
SS, if you want to get a little more elaborate with the system (not a lot, but it would require a little welding) you can make a jig that not only pre-stretches your strings, but will also hold them taut for serving, tying in silencers, etc.
I really appreciate the good word...after 20 or so years and thousands of strings, even I can't help but pick up a thing or two.
Chad