Do most companies back up their NetSuite data, or do you just trust that Oracle will not have a blunder and somehow lose some/all of your data? Should I think about occasionally backing up all the data in my NetSuite account?

I may be an outlier, but in 10yrs of NetSuite, I've never seen a client backup their NetSuite data outside of Sandboxes//Data Warehouse... which is basically what a back-up would be.

I have seen similar behavior as @mightyd3. Honestly, when you are paying THAT much for a cloud ERP, Oracle should take care of the backup.

How often would you be backing up data? NetSuite already provides mirrored data centers and periodic backup to magnetic tape. The multiple redundancies provide a pretty good - not perfect, but good - safeguard against blunders or disasters. Unless you're looking to do it daily, or at least several times per week, it's very unlikely you would be backing up at a higher frequency than NetSuite already is doing to at least 2 different locations.

And even if you were backing up the data, the ability to do anything with it would be negligible

I agree with @KevinJ of Kansas full csv data dump is a joke