I think thread weight is a personal preference.
With regards to piecing, I only use Bottom Line, which is a 60 wt thread. I tend to make quilts with small pieces and find the finer thread gives me more accurate seams (less bulk in the seam).
Regarding quilting, if you want your quilting to stand out, use a heavier thread. If you want it to quietly sink into the background and let your piecing sing, use a finer thread (such as Bottom Line). A plus to using finer thread is you can get away with a thread color that doesn't match your quilt. I have a friend who swears by pale blue, she says it blends in on almost all quilts.