A verbal warning would be situational at best.

If someone bursts into your bedroom at 2am, shouting a warning is probably not the 1st thing that will pop into your mind.
Alternatively, hearing someone creeping around down stairs at 2am is a completely different scenario.
It is a good thing to discuss, research and understand what the potential repercussions of a self defense shooting are and more importantly to be comfortable with them long before every purchasing a firearm for the express purpose of self defense.
At the moment of truth you do not need indecision keeping your finger off the trigger.

IB