David - just want to throw another suggestion out there as well. I personally back up the whole hard drive, or set of hard drives in my case, on a regular basis (weekly).
At least from my experience I never know when I'm going bump into something that will cause an issue to come up with the system. Sometimes it's just something annoying like a dialogue box that pops up when you boot up, and that dialogue box is for something suspicious. Sometimes it's very hard to determine why that annoying thing just started happening, but it didn't happen last week.
When that happens for me (and it does), it's amazingly cool to be able to go to the latest backup and restore everything. Problem solved after pushing a button and letting the backup restore everything.