I agree that it is vital to backup regularly - I'm amazed by people who don't do this - e.g. who have just one copy of their Ph.D. work stored on a USB stick!
However, to do it properly you should make sure that you have an offsite backup. If your only backup is local to your computer, it is as "at risk" as your computer. A break in, a theft, a flood, etc would not only mean you lose your computer, but also your backup.
The easy way to do this (what I do) is to have two backup discs - one of which is connected to my computer and one of which is at a friend's house. About once a week, I will swap these discs over - so if something happens to my local system, I know that I will have a backup which is a week or less old.