This is another "prewash or don't prewash" type of question. Some swear by a thinner thread on bottom. ...some say only same on top and in bobbin.
personally, I prefer my piecing threads to match (top/bobbin) in color and weight. I use a 50 wt most of the time, unless I need a color that I do not have in 50 wt. Most of my other thread is 40wt.