Prestretching works and it's recommended. Ever put a seam together and it's invisible until you stretch it? Tape is fighting the carpet because it stretches at a different rate. If ya haven't tried it, ya can't knock it. I've shown it before. After releasing the prestretched seam it does this. If you seam first then stretch, it fights just as bad as you stretch it. Seam doesn't stretch, but the the carpet does.