In my experience, I've never had a LiveCD hose my system just from running it. If you mount the drive or run a partitioner (like GParted) then you have a slightly higher probability of causing data loss. I've never had this happen though (unless I did it intentionally, anyway). I may have lost some data once when I tried mounting an already failed drive. But I had all my backups.
So I guess I would say you should always backup once a week, regardless. It's just good practice, even if you're not doing anything...