Depends on your fuel mileage towing, where you drive ( distance between stations) and how long you plan to drive without stopping.
The extra fuel gives you options as to where you stop and the ability of price checking along your route.
But if you stop for lunches, or only put on 350>400 miles per day between stops , probably unnecessary.
And the added weight of the extra fuel can put some truck / trailer combinations overweight.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.