I've got T-Mobile, but last year bought a new cellphone. In the instructions that came with it, the GPS was already "on", and you had to turn it off if you didn't want it. I read that you definitely want to have the GPS activated so if you are ever in a situation where your life depends on your being found, you can be located.
In November 2006, the Kim family from California went on a vacation to visit friends in Oregon. They never arrived home and there was a search for them. It took some time before someone did a cellphone search which showed a ping off a tower on a road that's not used in winter months by tourists. Once they found that ping, they started looking there for the family and found the wife and two small children, but the husband had left on foot to seek help and got lost. Searchers found his body a few days later.