Just as an FYI - warranty cards are often just marketing tools. It's a little different with car seats and other children products, because filling out the card allows the company to contact you with recall information - but you rarely gain any extra warranty protection by filling out the card (the warranty must specifically state that the card has to be returned to get protection). It's actually illegal in some states (I know California for sure) for a manufacturer to require you to fill out the card to receive the warranty.
Sending the card in when the product is received as a gift is sometimes helpful, because it can help date the warranty. Otherwise, the manufacturer may choose to start the warranty period from the time they shipped the product to the store.
I always fill out my car seat registration cards, but I rarely send any other warranty cards back. If I do, I only fill out my name and address, because answering any of the other questions is a great way to get junk mail. It's basically a way for the company to learn a lot about you - and then they often sell the information to other marketing firms. Another thing you can do, rather than send them in, is keep them somewhere safe in case you ever do need them.
Sorry if that got a little off topic!