Should C drive and My Documents back up at same time?

I'm using Ghost 15 on Windows XP. When I first installed Ghost, it prompted my by default to create a backup for the entire C drive plus the My Documents folder. I didn't see a reason for that and separated it into two backups at different times. But now I wondering: is there some relationship between the C drive and the My Documents folder that makes it advisable to make them part of the same backup?