My whole life long I have run my car until the gas tank was nearly empty. I have almost never run out of gas. I know my car's gas tank capacity better than the folks who designed the thing. I ran out of gas once in 1988 and once in 1997. Both happened because I had no money to refill the tank and was pushing my luck.
No one has ever been able to convince me that it saves time to fill up at a quarter of a tank or three quarters or whatever crazy time they refill. I don't buy the sludgy bottom of your gas tank is bad for your engine argument either. The other day a friend of mine was teasing his wife about it and I defended her, because seriously, it saves so much time to only refill when it's empty. Then someone mentioned that gas pumps are in the fuel tank and that they are designed to be cooled by the gasoline in the tank. If one runs a car on a near empty tank, the fuel pump is too hot regularly. In short, the habit wears out your fuel pump faster.
"HOGWASH!" said I. Then I began to research a bit and guess what. It's true. I was wrong. Okay, I'll say it louder. I was . . . um . . . wrong.
Since I have taught my empty gas tank falsehood far and wide, and in the spirit of full disclosure, I must now repudiate my former sentiments. Even though it takes longer and the sludge in the tank thing is silly, you really should refill your gas tank before it gets below a quarter of a tank so your fuel pump doesn't receive undue wear and tear.
I know that keeping the tank full, like getting regular oil changes and doing recommended maintenance, is a financially stable person's privilege. (Trust me, I know.) But still, if you can, it's a good idea. You know, when you get a car.