Welcome to Nikonians! Congratulations on your new D5000!
>A. Does Nikon recommend (or from your experience is it good) >to have routine, yearly cleanings of the camera body? How much >do these typically cost on their own?
No, I have not seen anything from Nikon recommending yearly service. I only clean the sensor when it is needed. The less you touch it the lower the risk of damaging the low pass filter. The price for filter cleaning ranges from about $30.00 to $75.00 or so depending where you go. About $50.00 or so would be the average price. If Best Buy will be doing the service, get a price from them for a sensor cleaning.
>B. Has anyone had issues with the extended warranties offered >by sellers, particularly Best Buy?
Never bought one!
>C. I'm open to opinions on this, so please feel free to >answer. Any feedback is appreciated. I'm usually not one for >these warranties, but I don't want to replace a $700 camera >out of pocket either.
If they cover accidental damage and you have small children??? My nephew got to my sisters camera breaking the lens mount and the shutter release button on the body. My daughter never touched my gear when she was little so it is your call as to the worth of the extended warranty! Good Luck and Enjoy your Nikons!