The car seats that may be best for risk reduction in the event of the worst kind of automobile accidents may be rear-facing seats with a grounded support leg and anchors to the car. Those kind of car seats most frequently don't work in plane seats on common carriers and would need to be checked in somehow.

If checking in car seats, best to have them boxed. But checked luggage doesn't mean the luggage necessarily makes it on time and in great condition for use on arrival.