Everyone has different needs. As noted above, my jerry cans are usually empty unless there is a reason to fill them.
Here is one example of a situation where carrying additional fuel is not only prudent, the failure to carry additional fuel would be risky at best:
In December and January I will be traveling the length of Baja California. One particular stretch on the route back north is 225 miles between Pemex stations. There is no cell service. No AAA.
19 gallons x 16 mpg = 304 miles, plenty of gas on paper. However, 16 mpg is highway mileage on a good day. Not every jeep gets that mileage and the fuel mileage threads on the various jeep forums contain numerous posts from jeepers who regularly get only 12-14 mpg in their TJ's. 14 mpg nets only 266 miles and 12 mpg only 228 miles, i.e. fumes as one rolls into that Pemex station after 225 miles. What if it is slow going or something goes awry? What if that Pemex station at the end of the 225 miles is out of fuel as often happens during the holiday season when so many Mexican families and American tourists are traveling? The next Pemex station is 60 miles farther with no guarantee that it will have fuel if the first one doesn't.
In this situation it would be foolhardy not to carry at least 5 gallons of additional gasoline. [And some cerveza.
]
Su experiencia puede ser diferente.