Lawns don't need fertilizer sure - if you don't mind weedy, thin, yellowish grass. If you want a good-looking lawn, follow these simple directions:

Mow high. Everybody around here scalps their yard thinking they won't have to mow as much when we have 20 hour daylight. The usual red fescue/kentucky bluegrass lawn around here should be at least 3 inches high when mowed. Never mow more than 1/3 of the grass blade at one time. 

Most of the time, use organic fertilizer. Milorganite is probably the best out there but it's expensive and hard to find up here. Organic fertilizers don't burn and they don't cause excessive top-growth, and they're just better for your soil. 

The one time to use synthetic fertilizer is right now. As temperatures cool, grass will start converting nitrogen to carbohydrates to store for the winter. A shot of quickly available nitrogen around Labor Day is very helpful for strengthening grass roots and keeping winter kill to a minimum.