If you fill the tank every time, you’re minimizing your time at the gas station. But if you always ask for $10 (remember when that was a lot?) you might save money. Asked to fill, the attendant may squeeze extra drops of gas onto the ground. Someone has noticed that the attendant may also misunderstand (?) you and fill with a higher octane than you requested. That could net him money if he’s filling the tank, but there’s little incentive for this trick if you asked for $10 worth.
Oh I’m so lococentric: if you don’t live in NJ or Oregon you probably fill the tank yourself, and my hairsplitting means nothing to you! How do YOU decide whether to fill the tank?