If for no other reason, I like to use 50 wt on the bobbin because I don't have to change the bobbin as often with it. My machine doesn't care if the top and bottom thread are mismatched, so no problem if I want to use a 40 weight on top for quilting. However, so far I haven't particularly felt like my quilting needed to be "showcased". More like concealed as much as possible, lol.