It is a personal preference. Aurifil 50wt is a 2 ply thread so it is a pretty fine thread to begin with. If you are happy with your results, by all means stick with the same thread in top and bobbin. My personal preference for piecing is Aurifil 50 wt on the top and Superior bottom line which is a 60 wt 3 ply poly in the bobbin. The main reason I use this mix is economical as the bottom line is less expensive than the aurifil. Additionally I only piece with two colors of thread. I use a beige/gray, kind of putty color for lights and a dark gray for darks. I do not switch thread colors to match fabrics or piecing.
I am also happy with the precision of my piecing with this combo. The reduced lint build up in my bobbin area is an added bonus but I find I get plenty of lint from the fabric. Especially if I am working with flannel.