I have used a very narrow zigzag. IMO it's best to use some kind of a stitch with "give", since the fabric is so easily stretched. If the fabrics stretches and the stitches do not, the stitches will break. A very narrow zigzag provides some give, but still gives the effect of a straight stitch.
Isn't a stretch stitch basically a narrow zigzag? Never thought to check my sewing machine to see......