I quilt on a long arm....and I usually match the bobbin thread with whatever color I am using on the top. I do usually use So Fine by Superior....but sometimes I use OMNI which is a thicker thread on top...then I try and match the bobbin thread as closely as I can with the thread I am using on top because I can wind more So Fine thread on the bobbin. I also have used a neutral color in the bobbin.....sometimes, no matter what you do....a bit of the bobbin thread might show on the top....and I want that to be as inconspicuous as possible....of course if you are using very busy fabrics it probably doesn't matter much....I always take the thread color I am thinking about using and unwind a bunch and throw it on top of the quilt top to see what I like most....I generally end up picking out a color that blends in the most with the lighter colors....but not always....I just don't like doing a lot of thread color changes and 95% of my quilts are utility quilts....