If you don't sand the whole panel the areas you did sand will have a different look when you re-topcoat. This can be rectified by color sanding the whole panel (after the new paint) to bring it all to the same texture before polishing, but, if you have to sand it anyway, do it to the old paint just to promote adhesion. An extreme example would be sanding the texture off a plastic inner panel in a couple of local spots then spraying the panel with paint. You WILL see those sanded surfaces as having a smoother "texture" then the areas you didn't sand.
Also feather edge the areas 4 or 5 inches all around. If you did a LOT of localised sanding, especially if you are down to primer, you may end up with warbles in the surface or edges that lift when the new paint is sprayed. I have gotten around most of this by spot spraying the sanded areas to bring them back up to the existing paint surface and then blocking them to level everything back out. This also helps seal the edges to keep them from lifting due to the new paint slightly melting them during the spraying process.