I stopped sending in the warranty cards years ago as a way to reduce junk mail. Instead, I keep them with the manual of whatever product I have purchased. Only once have I needed to actually use one of the warranties. We had not sent the warranty card in, but when we called for service, all we had to do was pull out the warranty card and give him the unique number on it and we got the part we needed for free!
I hesitate to suggest what you should do with your warranty cards - but I HIGHLY recommend this very well researched and informative post by Jeri Dansky about how warranty cards are used and the risks of not sending them in.