Planishing can take care of some problems. Try MPandC's demonstration (see the sticky thread up on top of the P&B forum) of welding two strips of metal together with zero gap and watch them overlap as you start from one end and put dots spaced out along the weld. Planishing expands the dots so that the overlap disappears. You might be able to get away without planishing but you'll likely wind up with a valley along the welded seam, especially along a curved panel patch.
