Not sure this Apple ''clerk'' at the store knew what they were talking about. If you download either gfxCardStatus or GPUInfoMenu, you will be able to see ''live'' which is being used--the Integrated graphics on the Intel chipset or the NVIDIA (also called discrete) GPU.
With gfxCardStatus you can actually override the settings and, for example, set only the Integrated to be used on battery--which--saves battery life.
Anytime the NVIDIA GPU is utilized, battery life will suffer. You will get much better battery life on either dynamic switching or the Integrated alone.
HOWEVER, CERTAIN OFFENDING APPLICATIONS WILL ''HIJACK'' THE NVIDIA GPU AND NOT LET IT GO. Firefox (I'm using 6 right now), for example, is the chief offender. Firefox will grab hold of your NVIDIA chip when playing flash--AND WILL NOT LET IT GO. So, once you've gone to a site and played some Flash content, you'll be stuck with using the NVIDIA GPU until you quit Firefox. You can fork over 99-cents to the App Store and get a little utility called FlashFrozen, set it to auto-kill Flash processes in Firefox and this helps--just be aware that the auto-kill will do exactly that--kill Flash processes. If you go to a site that wants to play Flash, sometimes it will just fail out.
LASTLY, Google+ and Hangouts: Again, in Firefox, if you initiate a Google+ Hangout, if you don't have it installed already, it will want you to install two plugins to enable the video part--and once again--if you initiate a Hangout, the NVIDIA GPU will be hijacked until you quit Firefox.
Long story (sorry) short: Until the various applications that hijack the NVIDIA GPU learn how to play nice, your battery life will suffer.