Having got a queue number at the Nikon Service Center, I decided to clarify again in person (I am on leave again) what actually is needed to keep the Nikon D90 at its peak condition.
According to the Nikon technician, you are advised never to clean the sensor on your own as it scratches quite easily. And a replacement of the sensor due to such act of ignorance is not covered under Nikon warranty. The warranty also does not cover wear and tear parts but does cover the hand grip if it ever gets sticky due to years of use. A wet cotton pad to clean the hand grip of my Nikon D90 always turns blackish every month.
The standard cleaning procedure went on quite professionally and quickly for my Nikon D90 today - without charge. The viewfinder was also thoroughly cleaned with a huge blower mechanism.