Didn't register mine because of privacy issues and because I sideload books from my computer. I use calibre to manage my collection and convert epubs for use on my Kindle. If not registered, you can't use collections, syncing with other devices, cloud storage for Amazon-purchased items. You avoid the special offers if you don't go online.
For me, collections were necessary. Fortunately, there is a jailbreak and a separate collections manager that works well. There are also some patches available that further enhance the Paperwhite's reading experience. By not going online, you avoid any automatic firmware updates that would disable these.