nah, you don't have to register.
the moment you turn on your Mac. The warranty starts and is already in the system.
Wah don't sabo, there's auto-enroll and there's separate purchase.
"If you purchased the AppleCare agreement at the same time as your Mac, iPad, iPhone, iPod, Apple TV or Apple display from the Apple Online Store, the Apple Retail Store, and select Apple resellers, your AppleCare agreement may have been automatically registered. If so, you will receive a proof of coverage, either immediately in the form of a receipt that indicates the end date of your coverage or within thirty days in the form of a Proof of Coverage certificate provided by Apple.
If you purchased your AppleCare agreement and your covered product separately, or if you purchase your hardware from a reseller who does not automatically register your AppleCare agreement, you will need to register."