I've used cotton for piecing in my Bernina, though now I prefer using a thin poly thread (Bottom Line) because it takes up less room in the seam and doesn't produce lint like cotton threads do. (I still get lint, but much less since it's only from the fabric.) If the machine's previous owner thought Berninas didn't like cotton it was probably because someone told her that, or she tried it once and had problems. It's a myth that Berninas don't like cotton, just as it's a myth that you should only use cotton thread with cotton fabric.