It is optional ... for warranty service, you still have to send in the receipt and warranty card - same for Sigma, Tamron, etc. So, its not like Apple's registration of their extended warranty, where if you register, you can just take the product to an Apple store and they'll verify that it is under warranty by checking the serial # of the product.
The warranty card is primarily a way for Canon (or anyone else) to gather marketing information. By law you do not have to send in the warranty card to be covered by warranty. You DO have to have proof of purchase showing a purchase date. Online registration is a good way to go.
