In your case you would want to run apt-get update after adding a PPA. Ubuntu automatically checks for updates either every week or as you configure it. It, when updates are available, shows a nice little GUI that lets you choose the updates to install, and then downloads/installs the selected ones.

Is it good to update Ubuntu?

You benefit from getting the latest software, including new security patches, and all the upgraded technology that comes with a new release without having to resort to reinstalling and reconfiguring your system. Best of all, upgrading Ubuntu to the latest release is easy.

How often should you apt upgrade?

I would run apt-get update; apt-get upgrade at least weekly in order to get any security patches. You should get little no upgrades on 14.04 that aren’t security related at this point if you only have the default repos setup. I wouldn’t bother setting up a cron job; just run the commands once every few days.