Some packages have the low fuel light, other don't. There is also not a hard and fast rule as to when they will light up. However, it seems to be from 2-4 gallons. Mine is pretty close to 3 gallons.
There is no reserve gas tank in our truck, or any other vehicle unless equipped with a dual tank setup. The reserve gas is located under the 1/4 tank mark on the gas guage

. Why stretch it? Like was already said, when you start consuming the gas at the bottom of the tank, you are sucking up all the deposits that have accumulated in there over the years. Good rule of thumb, fill up at the 1/4 or 1/8 mark.