You don't ever want to leave the part or panel structurally unsound. That's form over function, and doesn't belong on an automobile. As soon as my grinder/sander hits parent metal beside the weld, I quit grinding or sanding. If there is some slight low spot/crater from welding, I leave it for filler, weld again to fill the crater, or paint it. It all depends on the location, material, and function.
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