Unless you are making your last flight ever, what consequence, beyond a bit more weight from fuel, would it make to carry less fuel? It isn't like it will not get used on the next flight.
In my car (before I bought a Tesla), I always filled the tank to full. Unless you just like stopping at gas stations and pumping gas, filling up the car makes more sense. Theoretically vehicle weight plays into MPG ratings otherwise why would auto makers work to shave ounces and pounds off of a car's weight to the point of leaving out the spare tire (safety much?)
The point is this, on an annualized basis, would you really be able to tell that an airline saved X number of dollars by not putting adequate fuel in their tanks? I would guess no, considering all of the other variables that come into play.