We have officially Jumped The Shark on the stretch discussion. How about the answer to the question of mono/fluoro stretch is both true and false. I don’t know which one stretches more out of the box or how the force to stretch differs and it shouldn’t matter…you only get one out of the box experiment because of minimal rebound with fluoro. The first time you set a hook, fight a fish, pull a bass from the slop, hang a limb, you are going to stretch your fluoro. That stretch sets a new baseline for the amount of stretch you will get moving forward. Check it yourself. Wrap some fluoro around your hands and give it some stretch at different pressures. The more you stretch, the less and harder it is to stretch after. You eventually get to a point where it doesn’t stretch anymore (well before the breaking point). So IMO, fluoro does have less stretch after the initial stretch but somewhere close out of the box.
Does that mean the line becomes useless after the first stretch? I don’t know but I do know that I don’t re-spool after every fish, limb, etc. and I rarely break off. There likely isn’t a person on this site that has fluoro spooled on a reel that hasn’t been stretched.