I wouldn't send to the 645 mission that rewards you an epic item at 30% because I am not sure when you get the chance again to do that.
But if the rewards is only gold I would send followers no matter the percentage. There is really no drawback to failing missions, you even get the base xp anyway.
However spamming missions also costs ressources. It is important to use scavenger followers for ressource missions. Still you might run dry at some points when you upgrade all buildings like me, but treasure and rare hunt takes care of that and is actually content to DO and more fun than running heroic for the 20th time. Finding all treasures and rares.