I think that's probably right about the top 5 or 6 inches being the bit that has to collapse.
Only complication with stuff below that level might be if you had a real heavy landing and splayed the skids completely (al la the crash on Bribie Island last week) or stripped the skids off with a lot of forward speed and then had the misfortune to have solid rock/boulder/stuff directly under the cabin and have it push the floor upwards.
I've got a wrecked cabin in my workshop and the lower part of the seat boxes is concertina'd upwards as well. That would have pushed any contents upward with it.