when i shop at joann's i always add 1/8 yd because they notoriously don't cut it straight.
My experience at the quilting stores is that they do add a couple inches. And i've experienced the bow in the middles when using a rotary cutter - i've had to rethink that too....
I'm not sure i'd like the material to be torn - i do starch my pieces when i cut and put together