One thing that's for sure, don't fill out the warranty card until you're sure you want to keep the lens, since that will likely make it difficult to return. If that means ignoring Nikon's 10-day window, then so be it. That said, once you've decided to keep the lens, there's no reason to not send in the card.
I won't make any guarantees on Nikon policies, but I did have a warranty repair done on a 2-year-old 70-200 VRII with a sticky aperture. I had never sent in the card, but after the failure made sure the lens was registered online, and followed their instructions to the letter on sending in copies of receipt and warranty card with the lens. They covered the repair no questions asked.