There was another thread about this yesterday where people wanted it to be automatic. The thing that makes this tricky is that there are situations in which people want to be able to control when they use it. Two examples:

  • Crit fishers: You multi-class into a fighter and have Action Surge. You crit when you Action Surge attack, but the game used your sneak attack during the first section of attacks.
  • Currently it seems the game applies all damage regardless of what actually kills the enemy. If an enemy dies to one attack outside of sneak attack, a player that is dual wielding may wish to save their bonus attack and sneak attack damage for another enemy.