Lawns need some fertilizing to grow well each year. If unfed, they'll grow sparsely, allowing weed seeds to germinate and flourish. Grass that's insufficiently nourished is also more susceptible to pests and diseases.
What your lawn doesn't need, however, is year-round, overenthusiastic feeding. In fact, some say the greatest threat to our water supply comes not from pesticides or industrial runoff, but from excess nitrogen applied to lawns by home gardeners and landscape maintenance companies. Extra nitrogen fosters algal overgrowth in lakes and rivers, thus upsetting their ecosystems. Beyond this, it doesn't even help the lawn that much in the long run. More thatch develops, roots don't grow as well, and more water is needed.
Fertilizing just once or twice a year is sufficient for most lawns. If you're growing a cool-season grass, fertilize in fall; roots will be nourished and the stored nutrients will give new growth a boost in spring. Depending on the lawn's overall health and growth, you may want to fertilize again in late spring or early summer. Warm-season grasses should be fertilized in summer, at the height of their growth period. Fertilize once in June, a second time in August.
For any grass, use a fertilizer formulated especially for lawns, choosing a controlled-release formula. Some fertilizers are mixed with herbicides to control weeds in the lawn; such a product may be a good choice if your lawn is infested with a weed susceptible to the particular herbicide (check the label to see which weeds the product kills).
To ensure even distribution, use an applicator to fertilize your lawn. Uneven distribution often results in fertilizer burn or unevenly green grass.