I completely agree with this etiquette, but be careful. I once took a seat in the back to get an express Jumpseater on. Just prior to closing the door, the agent comes on board and tells me she needs me to get off immediately because the revenue pax showed up for my seat. As I’m leaving the plane the express guy is standing up front shaking his head and says “sorry dude”. I’m like, excuse me? He actually expected me to get off my own metal after doing him a favor and get left behind. I don’t think so. I’m all for helping a guy out, but if they try to unseat me at the last minute I’m taking the jump unless of course the captain doesn’t allow me to.