I'm probably being a bit more laissez-faire than you. I still back up my files, but I don't worry so much about converted files unless it took a lot of work to convert them. In my mind, as long as I keep the originals I can always convert them again should I want to read the book again (I'm a great re-reader so it's quite possible I would want to).
In a general sense with regards to backup, I back up the files that I would never wish to lose and would not be able to recreate in any way. I usually imagine 'what if I lost it in a fire?' When I put the idea to the test, it's quite amazing how little on my computer that is not expendable. Though there are still a lot I'd love to keep anyway. So I back up all of that
