Satin stitch edging

This edging came from Terry Grant's blog - I've used it a lot.

As you go round the trimmed edge with an open zigzag stitch, sew down a cord - one on each side. Try not to catch the cord in the stitching. The idea is that you can pull gently on the cord to tighten the edge slightly, to minimise buckling.
Here are the ends of the cord, ready to be pulled a bit tighter to firm up the edge of the quilt.

Note the little threads sticking out - these need to be trimmed back before stitching. Trim the cords just before you get to them with the stitching.

And I usually go round the edge of the batting with a felt pen of a suitable colour, matching the fabric or the thread.
The settings for your stitch may vary with the thickness of thread you use - it should completely cover the edge. (The corners can be a bit tricky.)