Lawns are where people and pets play outside the house. So applying chemical fertilizers is harmful to the ones we love. Consider using non-toxic methods and products, and alternatives to grass such a food garden.
Lawns absorb water, which helps reduce storm runoff and improve water quality. So keeping grass height between 2.5-3.5 inches is optimal.
Lawns have a significant cooling effect, provide oxygen, trap dust and dirt, promote healthful micro-organisms, prevent erosion and filter rainwater contaminants. So make sure to test the pH of your soil and aerate your soil regularly.
King County Natural Yard Care webpage: http://your.kingcounty.gov/solidwaste/naturalyardcare/
Seattle Public Utilities: http://www.seattle.gov/Util/EnvironmentConservation/MyLawnGarden/LawnCare/index.htm
Growing Food in the City: http://www.seattle.gov/util/cs/groups/public/@spu/@conservation/documents/webcontent/02_013331.pdf