Whether you should register is largely subjective, do you care about privacy & jailbreaks? Or is access to Amazon "collections" more important to you?
Registering gives you collections, the ability to email other books or documents to yourself to Amazon for delivery to your device and complementary invasion of your privacy. Amazon 'will track what you read, when you read and other big data, spam you accordingly and at least in the past, occasionally delete a book that they don't think you should have.
I'll stay unregistered, thank you. And wifi off as well (extra protection for my jailbreak). I can easily sideload books, that is transfer them from my computer to my device. You can get collections without registering if you jailbreak. But seems a moot point to me. Calibre will manage collections far better than Kindle ever will. So I let Calibre manage collections on my computer and just transfer a modest subset of books to my device for near-term reading which negates much need for collections on the device.
More valuable is adding useful metadata that you can use when searching for a book on your device. A huge number of books on Kindle just consumes battery as Kindle will try to index the entire text. And it's simply easier to navigate a reasonably sized subset of books on your Kindle. You can still interact or buy books from Amazon if you wish from your PC. If you decide to jailbreak you gain the ability to customize a lot of features e.g. screensavers and use alternate readers such as the inestimable koreader.
YMMV. If you want to register, you could "fake register" (see forum for details) or use a throwaway address as suggested by another poster.
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