If you plan on flashing over the internal OS, there is no need to register. When I got my nook, I made a bootable sdcard with CWM, booted into it, mounted to my PC, transferred CM7 to the internal storage and flashed it. From there I eventually flashed CM9 and only saw the B&N OS when I tested albert's nandroid backup.
tl;dr no need to register if you plan on flashing a rom.
Device: Samsung Galaxy Nexus
Android Version: 4.1.2; AOKP Build 5
Device: Motorola Xoom WiFi
Android Version: 4.1.2; Eos Nightlies
Device: Samsung Moment
Android Version: 2.2.2; UberMoment 1.3.4
Other devices I am familiar with:
HP Touchpad | HTC Droid Incredible | Motorola Droid X | Motorola Photon | Nook Color | Nook Tablet