Everyone should back up their files. How often is up to the individual and what you need. 
I'm going to mention a site I've mentioned before..... but www.myotherdrive.com is one you really shouldn't do without. Manual or automatic backups are a breeze and unlike some of the other storage sites out there, this one lets you choose how and what you want to backup. I've seen some services where if you delete it off your computer it automatically deletes the file off the server. What good is that if you make a mistake and delete the wrong file only to realize it later??? The whole point in backing up is to prevent situations like that. Then there's another one that has you backing up files based on where they come from so you are constantly having to keep up with multiple files if you have several computers like I do. What a pain. This is so much easier there's no reason not to back it up- SO BACK IT UP ALREADY!!!!