I've had this feature in all of my previous GPS's, but rarely use it. As I recall, I've use it just to play with, or to see what it actually does.
None the less...
I would think all the runway centerlines should be extended, no telling which runway/airport a person will choose. I hope it would only extend the runways at the airport in the flight plan.
There should be an option to turn one or more runway extensions on or off. Maybe through the airport information page. A button that would be visible directly from the main page. Push the button, find out the airport information, as well as the option to turn on or off one or more extensions. The only problem with accessing it through touching the airport, would be if I wanted to set up prior to the airport appearing on the screen.