we have had a couple of no shows this year,
but when i called and asked them what happened
they just said the reservations dropped off after 12 hours and we HAVE never been charged ( they did say if your flight had been delayed it is a good idea to call reservations to give them a heads up )
thus, everyone's mileage may vary, there does not seem to be a hard and fast policy regarding charges for NO Shows
it also might relate to your tier level
looked all over their site today looking for the policy but did not seem to find it
however, did find if you are EC member they now have a section called My Itineraries which shows all your active reservations with National, which should make it easier if you book again but do not cancel when they have a rate drop