I always laugh when I hear this question because there is not one answer for everyone. For most people having extra gas is a waste of space, if you stay on the main road and watch the fuel gauge and drive from campground to campground. BUT if you like to explore the back roads then carrying gas is smart.
I live in southern Alberta about an hour from the US border. Every year thousands of people come up from the US to go to the Calgary Stampede two hours north of here. Mixed in with all these people you will see some people that have their vehicles outfitted like they are on a world expedition; extra gas (some as many as four cans), extra tire or tires, snow shoes, sand ladders, etc, etc. One year I was working in a place that I got to see these vehicles coming in and I was curious so I asked them where they were from and where they were going, I was hoping for answers like "We are heading to Alaska" or "We are on a world tour" but what did I hear from most of them "We are from LA and heading to the Calgary Stampede then heading back home" or "we are from a place a couple hours south heading to the Stampede then back home". I was shocked as they were carrying more gear then they would ever need for 100 of these trips. Yes a few told me they were on their way to Alaska but these people had less than the ones that were just going from city to city.
I am one that fills up early, if I am at half and I see a gas station that is easy to access then I will pull in. If I am heading into the back country for a few days I will carry extra gas. But honestly if you are going from town to town or campground to campground you really don't have to.