I try and keep the originals plus copy's of all the converted format books in my library archives. One never knows how the tech will change over the years, plus self inflicted goof ups.
I have a 80 gig hd that came out of one of my dead laptops converted into a usb ext drive. That is used exclusivity for e-book/audio book storage and is not part of any system/file backups. I really do not trust any backups because over the years backup/image files have failed, and poof the info is gone!
(Think of the banker in the show, South Park and the banker and savings)
My electronic library is part of but also kept separate, so if disaster strikes I will have a copy saved that will help recover what is lost.