I'm one of the prewashers you were warned about! No meddling here, though, just my opinion. 100% cotton does shrink. If I am working with yardage (which is most of the time), I wash. If I'm working with smaller pieces (smaller than a FQ) I don't prewash or even soak. I DO however, starch everything before I cut just for stability.
And yes, when you prewash you will normally get tangled edges. I always over buy by at least a 1/2 yard so I have room for cutting errors or other boo boos along the way so having to square up the edge before I start is no big deal to me. Plus I'm more comfortable knowing that I'm closer to being on the straight of grain when I cut and will thus have even more stability with my piecing.