I will not blanket refuse to do it, depends on the circumstances.

Bumping a commuter: no
Helping a nonrev: more likely
Helping a rev pax: not quite as likely
DH to start work: less likely
Long flight: less likely
I'm tired and want my seat: no

I'll always run it by the captain too. If he'd rather have his elbow room than an extra pax, so be it.

I'm not going to leave anybody behind just on the principle of the thing. Use common sense.

Our company policy actually allows them to require us to do this on our own iron. But it may result in a fatigue call later on...