If you need to create a super stretchy cast on or bind off edge (like for toques or lace projects), use a larger needle to do the work for you. Similarly, if you have issues with your edges being too tight, try this technique too. It looks weird and feels strange but it works! Once you are done casting on/binding off give it a stretch, the extra yarn slack from each stitch will make the whole edge bigger than otherwise. Experiment with needle sizes, I like to go up about 4 sizes or so.