How much better would it be to kill the surround grass when planting a conifer tree hedge?
Not necessary. The dense shade provided from growing confier will eventually kill grass. Most conifer roots systems are extensive and go very deep and thick into the earth as tree grows. Be careful not to plant near sewer lines or foundations, as they can become a problem if planted near these areas. Raised beds aren’t absolutely necessary and will not influence growth. When initially planting, you do want to plant conifer root ball an inch or two above soil line so you don’t have a standing water risk. Insure soil surrounding conifer is loamy (fertile, yet will drain moisture). Soil that contains too much clay will absorb too much moisture and may kill a newly planted tree. Soil that is too rocky or sandy will not allow young plant to firmly anchor into ground and may roll over in a storm. I mix high quality topsoil (50-60%), native soil, peat moss, and some compost when I plant these fast growing trees.
