Although in most cases it wouldn't matter, pancit175 has a point. Depending on how much load you are putting on your ACC wire, and the guage of the wire, it could give you a somewhat lower reading that is not necessarily reflective of the battery voltage. As the draw increases on the ACC wire the voltage on that wire will be somewhat lower, even if the battery voltage has remained constant. Running a separate wire for the volt meter, and simply signaling-on the meter with the ACC wire would be more accurate. How much accuracy is required or whether this small difference is even measurable would be debatable.