No, they will not be using nvidia's discrete card. You're not alone in trying to find a solution for this, I'm currently trying to figure out a way that doesn't involve running
LD_LIBRARY_PATH=~/Steam/ubuntu12_32 optirun ~/Steam/SteamApps/common/$GAMEYOUWANTTOPLAY/$NAMEOFTHEIRBINARYFILE
in a terminal every time you want to run a game. On the plus side, it does mean you don't need to optirun steam at all, invoking that launch command will run the game itself with the discrete card regardless of what steam is running with.