Will a single crochet around the edge of stockinette keep it from rolling??
The stockinette pieces are the front and back of a bag, and the two side panels are not stockinette so I think that by seaming them together it will help a bit with the rolling but I’m hoping that something else will help even more.
Very often when I knit a sweater in pieces, the pieces curl up like a tube. After I seam them together, though, they’re fine. So I would think that the pieces that are curling now will be ok when seamed to the sides. 
I have used crochet to prevent the curling and it works fine. Depending on the weight of the yarn/garment you may need DC or TC instead.
I’ve been wondering how to prevent this as well. I seem to remember that there’s some sort of slip stitch method for keeping st edges from curling, but I don’t know what it is. Otherwise, 2 sts of garter on the edges should work too, no?
If you are knitting an edge that you are not going to sew, slipping the first stitch on each row can help prevent curling. A garter edge (usually about 5) will also prevent curling on an unfinished edge like a scarf.