My experience is.. my ground needs variable rate nitrogen.   However also based on my experience, and only because I've been growing corn with little to zero applied nitrogen for the past 8 years, To variable rate without knowing what N any given ground can produce, I would most likely apply way too much in the wrong areas, and shortchange the wrong areas.  (if based on #N per bushel rates)

This year for example I had a pivot that is half flat black and half hillside. The black bottom ground produced 280 bushel corn with less than 30 units of N, but the hillsides struggled to maintain much over 200 at that same rate. 

Had I not known that this high yield ground could produce 280 bushel corn with 27 units of N, I might have been tempted to increase rates on the high yield portion of the field, and decrease rates on the lower yielding portion.

Instead, the next time I have corn on this field, I plan to apply a higher N rate on the hilly, higher elevation but lower yielding half of this pivot, and maintain the low <30 rate on the high yield flat ground side. ***unless it is a very wet year.

There are also smaller sub areas within the hills that need other amendments other than N to equalize yield across the entire field.