It is generally a good idea to take one backup of everything once, so at least if your hard disk crashes then you can restore from this huge backup.
Make sure you know HOW to restore from this huge backup if your hard disk crashes. A backup is no good if you cant restore from it.
Then backup personal files from My Documents and so on on a regular basis (daily, weekly, monthly or whatever depending on how important the files are).
There are backup programs that can backup personal files on a regular basis, or each time they are changed.