I was told not to use cheap thread in my Bernina's. The cheapest thread I've ever bought is by Coats & Clark. However, when it comes to quilting or sewing projects I use thread I've bought at quilt shops on the fatter spools that is 100% Cotton.
I'd like to hear what other Bernina owners use and their experiences as I'm just starting with my recently purchased Bernina's and want to use what works best in my machines.