i dont think you should try and use almost all the petrol because the petrol at the bottom of the tank has all settled or something and is bad for your engine.
I dont let any of my cars get below 1/4 of a tank normally due to what 3styler has said - also I know how many miles I can do around town on the amount of petrol between 1/4 full and the red line. This way if I'm not near a petrol station when I would normally fill up I know how far I can travel assuming I'm not going to get stuck in traffic which is another reason why I dont run the car if possible below 1/4 as you never know when you might get stuck in a jam.
I wouldn't rely on the warning light. On a Saab 96 that I had the light used to come on when I had about 4 gallons left so I knew that I could do a good 100 miles or more before filling. Then one day...........it only came on when I had about a gallon left. I ran out of petrol twice before I twigged.