“Topping off” is never a good idea, but as long as you stop when the pump handle stops your system is quite capable of handling the thermal expansion. It is designed to accomodate the increase in volume due to both that and due to agitation. Agitated fluids increase in volume.
By the way, you can blow up a barn easier with a few inches of gas in a barrel than with the barrel full. Light a match next to the bung hole of a full barrel and flames will spew out until the gas burns off. Ligh a match next to the bung hole of a barrel with a higly concentrated mix of gas and air and you’ll get a free ride to the moon. Please don;t test this at home.