Typically the rental places charge more to fill it up for you if you bring it back with less gas than when you took it.

Sometimes I think it's worth it since it's difficult to coordinate the filling up close to the rental return.

What do you do?

I don't like that they charge you more than you would have to pay for the actual gas. I just go to the gas station nearest to wherever I rented the car. I don't mind filling up the tank even if it's really close already.