Default [Q] what data, exactly, are lost when flashing an updated stock firmware?

I'm currently STILL stuck with V10a-EUR-XX on my unbranded, non-rooted G3.
If I were to manually update it with a downloaded ROM, would I lose all my apps and their data?
Should I root, backup with titanium, then update and re-root?
Which tool should I use to flash the update that gives the best chances of maintaining data?

thanks!