I'm going to quote this study from Univ. of Illinois. http://web.extension.illinois.edu/state/newsdetail.cfm?NewsID=5814
Underdeveloped roots could the reduce the ability to recover/survive from diseases or drought. I do apply some nitrogen in the late spring (~first week of may). I normally go with a fast acting one to avoid feeding into the summer. Mid summer I might do half bag rate of Milo."Unfortunately, nitrogen fertilizer promotes shoot growth at the expense of the root system," says Robson. "Even with cool soil temperatures, the grass plant shifts growth to shoots to use the nitrogen. The root system stays undeveloped. We have been conditioned to expect a thick, green, lush lawn in the spring as soon as temperatures warm. Part of the problem can be traced back to the crabgrass killer combination products."