The more your IPD deviates from the 64mm lens position, the more tangential distortion correction is needed. Moving the IPD of the displayed images to match your eyes is not enough. Tangential distortion from lens offset (not viewing through the optical axis) is asymmetrical, magnifying one side more than the other, which causes a vertical disparity between eyes preventing stereoscopic vergence for portions of the screen. An offset lens position also causes significant additional chromatic aberration, making CA correction even more important.
For devices with adjustable IPD, radial distortion correction alone may be enough. For a device non-adjustable lens IPD (like the RiftDK), tangential distortion correction is NOT optional, and chromatic aberration correction is quite important too.
Another problem with non-adjustable lenses is that the distance between lens and screen is also not adjustable, making it impossible to focus accurately for some people. But even with focus, the RiftDK lenses rely heavily on viewing through the "sweet spot" (optical axis) to achieve a good focus.
The RiftDK would benefit GREATLY from replacement eyecups with mechanically adjustable IPD and focus (lens distance from screen) adjustments. But even with the standard optics that come with the RiftDK, we can do MUCH better with software lens distortion correction.
I am hoping that the "Interpupillary distance configuration utility, API" listed in the OculusVR "Upcoming Features" list includes tangential distortion correction and chromatic aberration correction. If not, I will be greatly disappointed...
For those who claim the added distortion corrections will require too much computation per frame, I say to stop doing any computations per frame and use a displacement (warp) map instead, which caches pre-computed pixels shifts, so that calculation complexity does not matter. People here have claimed that using warp-mapping instead of real-time calculation yields a 2.4x to 3.8x speedup for the pre-warp process. And such a warp map can include the extra distortion corrections I suggested above. I would reserve some of the pixel bits for extra red and blue shift though, for chromatic aberration correction.