Register Nikon: Purchase Date = Preordered Date?

Hi,

I'm on http://www.nikonusa.com/register filling out the registration of one of my cameras and lenses, and I have a few questions.

1) I preordered this particular camera on Amazon a couple of months before it actually shipped. On the online registration form, what date should I actually fill out for "Date of Purchase"? Should it be the preorder date, the date my credit card was charged, or the date Amazon actually shipped the product? Since there is a significant amount of time between preorder and when the camera actually got to me, I want to make sure I'm not misrepresenting & shortchanging myself, especially if it comes to the 1 Year warranty coverage period.

2) On the side of my lens, the serial number is labeled as "US123456". When registering it online, is the actual serial number of the product as written, or is it without the "US" prefix (i.e. simply "123456").

3) What advantages are there for actually registering your products with Nikon? Do you even have to register to get warranty service?

Thanks,
entlassen