As an example, I have seen some outer rocker repairs posted online that install less than a full rocker as the rust damage seen appeared to be confined to one end or an isolated area. The entirety of the rocker is held in with spot welds around the perimeter that in effect minimize the weld distortion if the replacement involves the entire rocker. Perform a vertical weld with the installation of a partial patch, absent any planishing because you can't reach the back side, and your result will be a guaranteed low area as the vertical weld pulls into a valley. Any effort you thought was saved by doing a piece meal job just gave you more work in final finish. I have also seen where many people suggest you install as little of the patch as needed to repair the rust. My preference is to repair what is needed to put the weld seam in the location that makes it most accessible for planishing, locates the seam where panel features best resist distortion, and do yourself a favor and leave the panel size as secondary to these considerations. You'll have a better repair in the long run.