Interesting point btw. If you call from home the 911 folks know exactly, precisely where you are. Not just your number but the physical address associated with that number.
Call from anything else and you're at the mercy of someone's GPS system. This is, I think, the same system that I periodically visit in the iPad just to see if it still insists I live in the parking lot across the street. There doesn't seem to be any way to send in a correction so they can fine-tune their data.
Besides, at least in this area: Call 911 from a "land line" and you get a trained 911 operator. Call from a cell phone and you get connected to, well, let's not name names but they are not the most highly regarded law-enforcement body in the country.
Moral: If you plan to have a heart attack, have it in your own home. Or, ahem, somebody's home. Or other fixed location.