The physical properties of the jack are such that contact between the pins on the cable head and the terminals in the jack are made almost immediately after inserting the cable head into the jack. They're engineered in such a way that they keep constant pressure against the jack, so that even if it moves up/down/left/right/in/out a little bit, the terminals and pins still connect.
Look inside one sometime and see how the 8 bits of copper are directed up at an angle toward the back of the jack.
Yes, if you wiggle it too far out you'll loose connection, but a couple millimeters here or there won't do anything.