I've made tweaks to both the prefs and ini and just want to know if Steam resets those back to the default settings when it updates the game.

It never has for me. I turned off the setting that tells it to set you to run on start-up and it's never reset that.

Though, if you changed more than just the basics, it may be a good idea anyways to refresh the INI files on an update in case something changed.

I agree with deaths_soul, I have not seen an update do a reset on the INIs. However, accidents can happen when editing (speaking from personal experience). To safeguard your tweaked files, keep a backup of them in a separate folder. I created a folder simply called Backup, and keep a copy of my latest tweaked INIs there. That way, I can compare any changes from an update, and also revert back to a good copy if I screw up the files.