My Bernina dealer basically told me if I don't use Mettler, Isacord, and Aurafil my machine will blow up (oddly, these are what they sell!!!). I haven't found that to be the case at all. I do use those threads, but I also use Dual Duty and Connecting Threads Essential. Essential is my favorite for piecing, Isacord for quilting and embroidery.
My machine has run great with every thread I've tried.