I'm another that saves my "left on bobbin" thread for piecing. My LA does not have a sensor, so it's usually only the very last bobbin I need to deal with. Since these are large bobbins, I rewind it to those for my main piecing machine and then use them when the color will work, though I never worry about a good match, mostly using dark on dark quilts and lights on light quilts.
I like to have about 20 or so bobbins for my main piecing machine, so I'll still have some empty if none of what is already wound will work. I've been doing a lot of scrappy sewing lately and have managed to use up all of the odds and ends. I also normally wind several spools of a neutral color when I need to wind one (ran out). I won't do that for colors I don't know if I'll use often, but if I need to wind a bobbin of white or beige, I'm gong to wind three as I know I'll use them all up.