I always use the same thread in the bobbin that I do in the top, unless I'm trying to use up a bobbin. (Normally because I need to rethread it with another color & I'm out of empty bobbins!) I'm thread stupid though so I stick with the same weight for top and bobbin.
Geez, what I wouldn't give to take a class on thread and how to thread paint. Some of those professionals do amazing things, but I have a fear of thread. I know, sounds silly, but I just feel like a complete dunce when it comes to thread and how different types work with the tension.