I used to buy into that whole "stretching weakens muscles" crap... but that's what I mostly believe it is now.. crap. Ever watch Dmitri Klokov warm up before snatching 200kg overhead? He does a bunch of static and dynamic stretches. Once I got into my 30's, especially when I am routinely squatting 3-400+lbs, I stretch my legs and lower back before every squat day. It has helped improve my flexibility and mobility immensely, and actually helped my strength and squat depth. I always do dynamic stretching with the ROM for whatever exercise I start with, but I also incorporate static stretching now as well. May not work for everyone, but I definitely don't believe it automatically hurts your strength. Try it for yourself and see.