Must I install Google Apps again after installing a new ROM? When I install a new ROM (cm-10.1-20130213-NIGHTLY-mako.zip) using ROM Manager, I have the option of adding another ZIP during the same reboot -- this is probably meant for the Google Apps (gapps-jb-20121212-signed.zip).

What if I don't add that Apps zip? What will I miss?  

Whenever there's a new build that I would want to install, should I always also add this ZIP? Or can I skip it because I already added it last time?
If you don't install the Google Apps package the first time, you won't have access to anything that relies on Google's services framework. This means things like:


Google Play
Google Talk
Google Maps
The Gmail application


...etc, won't function on your device because they will lack the libraries needed to run.

You do not need to re-install the Google Apps package every time you update CyanogenMod. It will essentially back up your current ones and then re-install them during the flashing process. Simply flash the updated ROM however you normally would.