Mark Spencer's comment reflects my opinion also, although I have never returned a warranty card. I don't do it because I want to prevent my name getting on some mailing/email list I don't want to be a part of (ie. Privacy issues)
Also, I do have a personal belief that should my item break down outside of the standard store warranty period, it would likely be related to a case of abuse. Example: using a screwdriver as a prybar should NOT be covered under warranty. Torking an open ended wrench with a 2 foot lever should not be covered under warranty. Driving your car into a wall should not be covered under warranty. Warranty's usually protect the buyer from a manufacturing defect, which usually appear pretty quickly if you use the item right away. They key is to not leave the item in the garage for a year before opening the box, finding something wrong with the item, then taking to the internet to complain "the big store" won't honor the 1 year warranty. If people stopped trying to get free tools out of abusing their tools, maybe (and I'm not overly optimistic about this, I must admit) the savings to the company could be passed on to the purchaser...right? I know...unlikely.
Ever notice how the "Really really" expensive tool companies offer the best customer service (ie. give out the most easily honored warranty?) I hear all the time about Snap-On warranties being the best...no questions asked. But you're paying 2 to 3 times or more than the price the item is worth ("worth" is a difficult word to define, I know....I this case, I mean the dollar value a "non-branded" same tool could be sold for). The company can thus afford to replace easily no questions asked, while maintaining a great reputation...
But I digress...
The short answer is : I don't fill in those cards. Never.
I should have just checked the box, right?
Depends on the price of the item. So many times, it seems like I have ended up on other mail / email lists because I registered with one company.