So as I do it different, I leave them on the bottom. When I am finished, I cut the bottom threads where still connected halfway between insertion points into the fabric. I then take a eyeless needle and spend an hour (or whatever) burying my top threads, then an hour (or whatever) burying my bottom threads. The time saved not worrying about pulling bobbin threads up makes up that time. You were going to have to bury them anyway. I don't have top threads, nor bottom bobbin threads pull out of my quilts after wash.
tim in san jose