Well, it's really not the BEST thing for your car, and here's why:
On the simplest level, feeding your engine from the bottom of the gas tank is very similar to drinking the last cup of coffee in the pot: those coffee grinds and sediment that have settled to the bottom of the pot? Similar stuff has settled to the bottom of your gas tank, and if you send that into the fuel system, you risk clogging filters and lines along the way to the engine. That stuff can plug up the fuel filter, causing erratic acceleration and taxing the fuel pump. And as our mechanic friends have reminded us, fuel pumps are very expensive!
Sure, sure, the fuel filter is always sucking gas from the bottom of the tank, right? Yes, but when the tank is full, any sediment present in the tank is more likely to be swished around and up into the greater volume of liquid. As the volume of gas decreases, the percentage of dirt and other particulates increases and is more easily sucked into the fuel system.
We've also been told that allowing the gas tank to stay below 3/4 empty for long periods of time can also contribute to moisture condensation within the tank, which could also negatively affect the quality of the fuel. During winter months in especially cold climates, moisture in your car's fuel lines can freeze and prevent your car from starting.
So, whether or not it's an old wives tale, most people believe it's a good idea to try to keep your tank above half full in freezing climates and above a quarter full the rest of the time.
Plus, it's just better to be safe than sorry, especially if you're one of those folks who spends much time in bumper-to-bumper traffic!