previous steps before to calculate disparity? Is rectification needed?
Yes, disparity needs rectified images. Since the stereo matching is done with epipolar lines, rectified images ensure that all the distortions are rectified and hence the algorithm can search blocks in a straight line. For a basic level you can try out StereoBM provided by openCV using the recitified stereo image pair.
Raw frames from camera -> Rectification -> Disparity map -> Depth perception. 
This will be the pipeline for any passive stereo camera.
I want to do stereo vision and finally find the real distance to the objects from cameras. I have done image rectification.Now I want to calculate disparity. My question is, to do disparity, do I need to rectify images first? Thank you!