General :: Nandroid Backups Obsolete After ROM Update?

4.2 of Jelly Bean update could be imminent for my Nexus 7, and I'm wondering about what and how to backup before the update. The question I think is a general android one though... which is why I'm posting it here.

This was going to be a question about factory reset... but reading other posts I think I now understand that factory reset just deletes data/cache, so I think any time you do that it's just gonna return the device to a clean ROM state without changing the ROM. (whatever flavour/version ROM is)

When a ROM update is applied (either official or manually flashed), I guess it wipes everything, is it like a disk format or something like that? I have installed CWM recovery and rooted my device, I think the update will unroot it, but will the CWM recovery still be loaded?

And then, what backup should I take before the update... I think a Titanium Backup is a must, I understand how that works. But a Nandroid backup too? Or is that more like taking a disk image, would that be useless to restore after the ROM has been updated? And if so, does that mean any old Nandroid backups should be discarded?

Once the new update to 4.2 is installed, is it advisable to take a Nandroid backup immediately, or better to download the stock 4.2 image and keeping it as a starting point? (does this amount to the same thing?) I guess this is where a factory reset would return it to stock (providing I haven't moved anything to the system apps)?

Or is it possible/better to use a Nandroid backup as a point in time image of everything that's been installed on the device after adding back all the apps etc? I guess that leads me to wonder if the Nandroid backup is needed at all if I'm going with the Titanium Backups...