I agree with the concept of monitoring as many parameters as possible. In addition to voltage, I also monitor EGT, fuel pressure, engine bay temperature, vacuum, intake temp, boost, oil temperature, transmission temperature and engine elapsed time.
Rather than use individual gauges, I collect the data using sensors and an Arduino which then sends the data to an Android tablet over Bluetooth. I have a library of screen presentations with analog gauges, ribbon gauges, digital gauges...and even a simulated Nixie Tube set. The tablet is mounted in the horn pad, so it's also available for navigation and a HUD showing speed, direction, altitude, elapsed time, average speed, etc.
Here's one screen.
