Most login-managers (e.g. KDM and GDM) skip .xinitrc and .xsession when you specify a premade session.
A solution to this could be to set the session to default instead of KDE, and put startkde in .xsession together with nvidia-settings. Note that if you want to change desktop/wm, you have to edit the script.
As another solution, make a shell-script in ~/.kde/Autostart to launch nvidia-settings. Note that this will only work in KDE.