It is essential to use a twin needle when you are hemming a knit. The back of the stitch, (on the underside), is indeed a zigzag and this is what gives your garment the "stretch" it needs so that your stitching doesn't break when stressed. As for pintucks, I love pintucks. There are several different feet that give different looks and several different widths of double, or triple,needles to acquire those looks. You can use pintucks on the yokes of little girls dresses or the yokes of nighties. Several rows of serpentine pintucks run vertically down a yoke, using one of your machine's special stitches are exceptionally pretty. You just need to remember to stay within the limits of your pressure foot opening so that you don't break needles. I have often used pintucks as inserts on a pillows. People are amazed to see them because not that many people seem to use the technique anymore.