considering how much fabrics and threads have changed over the years, i don't think we truly have a good frame of reference when we worry whether or not poly thread will weaken cotton fabrics in quilts we make today.
for the record, i have one machine that performs best when i use the $1 poly from wallyworld. go figure. :lol:
i used to use poly because it was easiest to find unless i shopped for cotton online. i decided to order some cotton just to see what all the fuss was about. that's when i discovered that i can do something with cotton thread that i can't do with poly: if i need to unsew a long seam and don't want to spend time carefully undoing it, i can just pick out the first half inch and then rip the sucker out by pulling apart the two strips with a good strong yank. any resulting distortion of the strips can be cured with a good re-press. that's even easier if i cut and stitched the strips up (along the selvage) instead of across the width of the fabric.
given the amount of time i spend unsewing, i was sold on cotton for life. :lol:
i get solid colors from connectingthreads.com
i get my variegated from threadart.com