Hi Butterflywing,
To be honest with you I haven't yet quilted a King size quilt, but the frame is set up so that I can. Sometimes, though, I attach my quilt to the feeder cloths by the sides rather than the top and bottom. This gives me more workable space before I have to roll it, which I love.
I do leave a blank area on the sides so that I can check my stitches before I actually sew on my quilt. As far as filling the bobbin, I fill it as I'm quilting. I buy two of the same color thread and away I go. Eventually, all the thread is used up but it takes a while (several quilts).
I don't snip any threads on the bottom. I pull the thread up to the top, take a stitch or two with the needle in the needle down position, tie off the top and bobbin threads and snip the thread leaving it long enough(approx. 1.5 " - 2") to tie it and hide the threads. Using a self threading sewing needle (needle with a slit at the top of the eye) I weave both threads through the batting pulling the knot so it gets hidden in the batting. The self threading needle is a must, the thread just slips through the slit no need to try to put the thread through the eye the conventional way. I do the same when I end the motif, I pull the bobbin thread to the top, tie it off and hide it in in the batting.
I like doing it this way because I know it's secure and you can't tell where I started and ended. It looks professional, like I know what I'm doing, ha, ha! :lol: Hope this helps! Please let me know. :-) Happy quilting :!: