Any 30wt thread would be too thick for accurate piecing. I like to use 50wt 2-ply or 60wt or even finer thread for piecing. A 30wt thread will show up well for quilting. If you want your quilting to sink into the fabric and be less noticeable, you would go for a finer weight thread; 30wt is for when you want your quilting lines to really show in the finished quilt.