Should the shear pins be used if the flight is single deploy?
Generally No. Shear pins are used with DD to hold a nose cone on a payload when they hit the end of the shock cord after a vigorous apogee ejection. As long as you can pick up the rocket by the nose cone and give it a slight shake without the nose cone coming off, you should be able to fly almost anything single deploy without a problem. If you have a heavily weighted nose cone and large draggy fins you might consider shear pins on a single deploy but I would consider the need to be a rare occurrence.