Thread weight is a personal choice and often depends on the 'look' you are after. The only time I use 40 weight is when I want to really emphasize the quilting stitches. I often use 60 or 70 for quilting as I really like to see the texture rather than the quilting thread. I use 60 for piecing.
If you do decide to change the weight of thread you use for piecing, remember to check your seam allowance, as thread thickness may affect it:
http://www.quiltingboard.com/tutoria...ce-t89997.html
I attach almost all my bindings by machine and just use whatever weight & color matches the binding best.
Remember to use an appropriate needle size to match your thread weight.