What if your battery voltage is more then 6v. Would the gauge be accurate? How about if the resistance value in the sender changed. Would the gauge be accurate? The gauge needs to see whatever voltage it is designed to operate on. My point was, that a regulator provides a rock solid voltage for that it needs to be accurate. Resistors can and will change value for many reasons and will have an effect on how the gauge functions. Weather the system came with a regulator, my '64 Rambler had a 5v points type regulator from the factory, to our 6v MOPARS with no regulators at all, both benefit with a constant voltage source.
In the case of my AMC, the gas and temp gauges went crazy when the stock regulator was failing. A 7805 solid state reg put everything right. Resistors have there place, but not if a working and accurate gauge is important. BTW....these parts are a couple of bucks, put out little heat and take very little space.