Bias, always. At first I only did bias and rounded corners. But then I had to do a quilt that looked better with square corners. I already had the bias binding cut out so I used it, and found that mitering the corners is soooooo much easier with bias binding. So now I use bias 99.9% of the time with that .1% only being wall hangings when I want to use scrap jelly roll strips or something for binding.