It depends on if you want the thread to sink into the quilt so it barely shows, or if you want the thread to be a design element. I frequently use multiple weights of thread in a quilt. For stich-in-the-ditch, I generally use a finer thread, but if I am doing fancy quilting that I want to show I use a heavier weight. I always do a sample piece with the thread(s) I want to use, the batting (because the thickness matters) and play with the stitches/patterns until I am happy with what I am going to do.