Why you should backup your library
Ah, the time-old question. You’ll never really know the answer to this until something catastrophic happens, and you lose something you really needed. So just take our word for it, you need to. Take the three seconds to backup your library — and the rest of your machine, for that matter!
We also recommend that you enable Mac OS X's built-in "Time Machine," so you'll always have a backup of your entire system (including Delicious Library 2's collection). Look for it in System Preferences. If you don't have an extra drive for Time Machine, you can still back up from Delicious Library 2 itself — there's a built-in Backup function under the File menu. Use it!