I too have been taking my D600 into the dealer I bought it from for both wet and dry cleaning since buying it in Oct 2012. In that time, they have either wet or dry cleaned the sensor at least 10 times to my recollection.
This morning I took my D600 into them to let them send it in to my nearest Nikon Repair Center in So Cal. Along with it I included a letter detailing where purchased and the amount of times the dealer has cleaned the sensor.
I agree with many posts here on these forums, regarding self cleaning and maintenance of one's sensors, but felt in my case with the amount of repeated cleanings, I'd send it in and see what Nikon does with it.
I'll be interested in seeing what Nikon does, other than a routine sensor cleaning.