You should not top off modern car's tanks. It can do damge and can make filling the tank more difficult and may require a moderatly expensive repair. That is the reason the manufacturer tells you not to do it.
Your explanation about how much fuel you can get in there after it stops, makes we wonder if you have not already damaged it.
In any case, it does not cost you more to follow the instructions and not top it off.
Busted did a very nice explanation.