You won't need to worry about this. I have not sent in any registration card within 10 days, for the reasons you noted. Either I sent them in after a month, or not at all. I never had an issue with warranty. The registration card is used by Nikon's marketing, and sending it in won't help with warranty claims, and not sending it in won't hurt. For warranty, all you need is the warranty card and the receipt. They just give you that 10 day limit for the card so you won't forget to send it in.