i had a 99 4runner and i used to always fill it to the very to of the spout. then after a few years i stopped and noticed that when it clicked of my fuel indicator never hit the full mark. my logical guess was that the constant over filling had actually damaged the fuel sending unit in the unit in the tank. as far as causing damage structurally to the tank i am sure they designed the tank to handle it. i believe that the sign at the pumps was put out by the EPA. some dude that gets paid too much says that overfilling cause more polution. whether he or she is right is a mystery.