There can be exceptions. I did once to help out a poor girl. Original plan was operate a 2pm flight to EWR that cancelled. New plan was DH on the redeye that night. Because of the cancellation, of course the flights are oversold. For the redeye flight, I waited until all passengers were on. Then it was down to one college girl trying to get back to school and she had no seat because the last seat available was my DH seat. She needed to get back that day (and to be fair, she had booked the 2pm flight to give ample time to come back). There being no jumpseaters, I offered her my DH seat and I took one flight deck jumpseat. Everyone happy.