About the only time I've put my hand on my grip "for real" was with our neighbor across the way in our old house.
It seems he had three Rottweilers, and liked to herd two of them into his truck without a lead when he went to work. Sometimes they got loose and wandered the neighborhood. The other guy was kept in the back yard. My only exposure to him was a very deep seated growl when I would walk my mini-Schnauzer on the sidewalk on his side.
One day one of them surprised me in my yard as I was walking to the truck. I was a bit shook, and backed into the house with a master grip on my pistol. We called the cops because this just wasn't right. Saw a squad visit the house within 5 minutes for about an hour.
Not sure what the deal was with this guy, but apparently from other folks in the neighborhood, this guy lets this happen a lot.
Reflecting on the incident, I am not sure what would have triggered a shoot reaction from me. I honestly wasn't sure which Rottie it was, but I chose to back off on my own property, and inform law enforcement about it.
If I was caught on my property, with no available physical barrier, and had a large aggressive breed display hostility in front of me (focus on me, teeth bare, deep growl, large physically aggressive dog) I would go to low ready and try to slowly back up. I think a 'shoot' decision would be movement towards me, either a lunge or running towards me at speed.
I'd be interested in other perspectives. Shooting a dog does not thrill me, but circumstances would dictate my reaction.