For bed quilts, I pull the bobbin thread to the front when I start. Then, when I finish the quilt, I tie all the bobbin threads to their respective front threads and make a little knot, and then use a sewing needle to bury the thread into the top of the fabric. If you tug just a bit, the knot goes under the fabric, but because it is a knot, it can't work it's way back out of the fabric.
For wall quilts, I start with very small stitches and then work up to the size I'm working on; that is enough to secure the thread and I just snip the tails off.
I find the fix button on my machine to create too large of a knot.