I purchased a sensor cleaning kit for my 600 from Copperhill (no affilliation!) in order to begin cleaning/maintenance on my own. My D600 was really pretty clean, all considered. Most of the dust blew off with the standard rocket blower. I did need a swipe with a sensor brush on the mirror to rid it of a pesky particle, and several on the sensor itself. Spotless now! While at it I looked at my D7000 - I cleaned dust off the mirror and the left side of the sensor had many oil spots that I have now removed by wet cleaning.
Glad to have begun this oft neglected chore. These two episodes could not have taken more than twenty minutes.