If you are doing free motion quilting, the bobbins thread can pop up even if you tension is OK. I know in the beginning when Long Arm quilting machines became popular, many of them required the same color in the bobbin that was to be used on the top. I quilted with my Bernina so with straight stitching, I had no problem. But when doing free motion, I always tried to keep the colors similar so that if the bobbin happened to pop up occasionally, it would not show. It is near impossible in FMQ to be precise with exact stitch length. That is why the stitch regulators became so popular when they first became available.