Hi nustian116,
A rectified stereo image will produce an image where the eipopolar lines are parallel, and the epipolar points are points at infinity. This fundamental matrix is the same as a pure translation along the x-axis. So two images related by a pure translation in the x direction wouldn't need to be rectified. However, rectification is used because it is nearly impossible to create such a perfect set-up in real life. You can fine more information on page 247 of this document, and in these slides I found.
I hope this helps!
Emily C
Applications Engineer
National Instruments