I usually use the same top & bottom when Piecing, wind bobbins from the spool I’m using on top- but sometimes I am just using up bobbins. I don’t worry about things like that. If I want to get a lot of thread on my bobbin I use Superior Threads ( Bottomline) thread. If I am using silk thread or invisible thread ( the two I use most often to machine stitch appliqué) I use Bottomline or a 50-60 weight thread in my bobbin.