Computers are not infallible. I know that is hard to believe, (Ha, Ha, Ha) but they do fail. Since this is true, you need to backup your computer regularly so that you do not lose your data. How often you backup is based on how much data you save to your computer. Some people back up monthly, some weekly, some daily and some even hourly.
How you backup your computer (to tape, CD, DVD or online) is not as important as that you do it regularly. Once you have made your backup, you should store a copy "offsite." Meaning at another location. What good is it to have a backup that is at the same location as your computer if a natural disaster hits that location?