Does Bernina have a single stitch plate? If they do, and you piece a lot, it might be worth the money.
I have a 9mm Pfaff, but I use a 1/4 inch foot for piecing. It still has a bad habit of eating fabric on start unless you use enders. I bought the single stitch plate. Pfaff does have some 6mm machines. Single stitch plate still recommended.
An old trick from years ago was to take the foot off, put the needle down and put tape alongside the needle, making the zigzag opening more single stitch.