Every time you open the gas tank, you lose gas vapor that your car could use. So you should try to open it up as little as possible. Furthermore, a car that is low on gas weighs less, and it takes less gas to move it. There is no harm in running low on gas. There is only harm in running out of gas.
The trick is to wait till your tank is empty and fill it up. Don’t put the bare minimum in the tank. I know gas is expensive in these uncertain times, but try and fill your tank completely each time you go to the pump.
The more trips to the gas station, the more gas you’ll be using. Fill up your tank, and you’ll save gas and reduce emissions overall.
Source: Daily Fuel Economy Tip via Josh Peterson, Planet Green