the only time people are really stressing over their thread colors matching is when doing applique, or quilting ...the stitches show and (at least with applique) you want them as invisible as possible so a good match is needed. for quilting it is easier to see the design over the (thread) if it blends in.
for piecing i just use what ever neutral color is handy...unless working with black or very dark fabrics then i use dark thread.