Why would you ever not assign your full crew on a job?
The question makes no sense; the game has no concept of assigning crew to a job. 

That said, it is possible that some crew members are not available to Misbehave. For example, Shepherd Book stays on the ship for immoral jobs, River Tam may have returned to the ship due to an earlier roll, and a medic may have returned a crew to the ship to heal.