You should back up everyday. The only cost is a couple minutes of time and it is valuable insurance.
We do our own backups – in a different way, though. We use what is called a “replication server.” This is actually two distinct computers, in different data centers. When writes are done to one computer, they are also replicated on the other computer. If a problem happens with one computer, the other computer automatically takes over. The backup computer will re-build the data on the main computer automatically.
In practice, it works very well. Amazon’s servers are the most reliable we have found (we have used four other hosting companies).
Plus, having replications help when we get occasional requests like, “Can you get me a copy of my data from a couple days ago?”
We keep the backups around for a week and they are automatically removed.