You need it once on the Live USB (or CD).
Then you do "Install". When it installs it won't automatically add the nomodeset.
So on the first reboot from the newly installed disk, you need to stop in GRUB and edit in the nomodeset.
Then I installed the nVidia driver and this (apparently) removes the need for nomodeset on any subsequent reboots.
This is all in accordance with the instructions on the System 76 web site.