I'm so paranoid, I convert Amazon books, even though I am friends with Dark and Alf. Then I delete the Amazon version-figure I can always get that back from Amazon, although why I would want to, I don't know.
If you have and account in the cloud, there is a file on your computer with the books in it. Now, of course, if your account is deleted, it would probably wipe out the file on your computer as well. I just back up my Calibre library monthly.