I do all my piecing with a 70/10 needle, but I use BottomLine thread, which is 60 wt. I like it because it's not linty, and it doesn't take up as much space in the seam, so it's easier for me to get the right seam allowance. The needle size is determined by the thread, and the needle type is determined by other factors, such as the type of fabric. There are charts and articles online to help you decide what needle to use.