Knits unravel. This is not exactly breaking news to anyone who knits and has dropped a stitch. Suddenly, without warning, there's a hole in your work the size of the Grand Canyon, and panic ensues.
Yes, sergers are great for finishing all edges, and offer particular protection for knit and stretchy fabrics. But they're a bit . . . aggressive. With 3, 4 - sometimes 5 - needles working at once and a blade slicing up and down, it's easy for the machine to distort fabric.
My solution? Organic cotton twill tape.
I use it to secure the cut edges before I do anything to fabric that I've machine knit.
But just to be clear: at the END of the construction process I DO serge all of the edges! I don't consider a piece complete until the interior seams have been cleaned up and reinforced.