Depends on where I am:
Where I live, I know where the local gas stations are that I like to use, and I know they are never far away. Likewise, in certain other cities I where I go occasionally. In those places, I may let it go closer to empty. But if I’m traveling in areas I don’t know, I’ll start watching for a gas station of my liking about when it gets down to 1/2.
I have a strong preference for one particular brand because their prices are consistently lower than everyone else. Sometimes by just a penny or two, sometimes by as much as 0.25 (!) – I wish EVERYBODY would go to the cheapest station, even if it’s just a penny. That would force the gas companies into perpetual price wars. I know I can’t make everybody follow my utopian plan, but still, I do my part.
(I won’t name the company, but I’ll admit I quit giving them my business for a few weeks a while back, when they were busily polluting the Gulf.)
AND furthermore – my beat up old clunker has an unreliable gas gauge that seems to give semi-random readings, but only when the tank is over 1/2 full. Anyway, I reset the trip meter (the resettable odometer) at every fill-up and always keep a careful eye on that.