Do you top off at the gas station?

I used to when I first started driving, when I was paying with cash. As credit card payment became standard shortly after that, I stopped worrying about topping off. No real reason, I never thought about it. While I was getting gas this weekend there was a sign above the pump warning of the dangers of topping off. It looked like some sort of PSA campaign. It warned that topping off was a waste of gas and money and harmed the environment. I don’t know how the system works, but that doesn’t make any sense to me. I always figured that as long as there wasn’t gas overflowing, it was in the tank and would be used eventually. I don’t think someone would design a system where excess gas will just leak out.