Convergence

Because the user's eyes are doing all the actual converging, the cameras should be parallel (i.e. converged on infinity), and spaced at the IPD (unless you are doing some effect with avatar size). If you try and converge the cameras, you are essentially rotating the entire world in opposite directions for each of the users eyes. Artificially converging the cameras is an artefact of the fact that stereo 3D on a flat display has to deal with a small FoV and borders to the image, and marry that with the desire to cut between scenes and use artful shots. Doing this sort of thing in a HMD is a highway to nausea city, and maintaining orthostereo is preferred.