Robin,
It is over thirteen years since I used TANS and around seven since I last used a precision Navaid. (HINS)
It is my recollection that these aids if uncorrected would initialise to shutdown position. With a serviceable kit this would lead to no errors, unless the aircraft had been towed a considerable distance between sorties.
Every crew I have ever known, would have entered precise co-ordinates on kit start up. These would have been taken very accurately from surveyed pan position or a very accurate IN or landing chart.
Waypoint one is a navigation turning point. You can enter it as North or South Pole without having any effect on kit accuracy.