Metering for ND Grad
If you meter through the grad, technically you're changing the information the camera is metering. Instead of a bright sky, you now have a darker sky, so it's technically not the same. You can't be sure exactly what decision the camera will make. It's better to take control of the metering and settings, knowing how the grad will affect the scene. If you have time for a grad, you have time to meter.
Joel Nisleit Photography
A camera or lens can't take good or bad photos any more than a brush and canvas can paint themselves a masterpiece.