I'ld be sure that a dog was really actually attacking, and I didn't have other options, before introducing a weapon into it.
On the other hand, how can you be sure, I mean absolutley positivley sure that you are about to be attacked?
Truth is...you cant...until you have been bitten. If you get bit by a large aggressive dog, you may not be able to shoot it, or defend yourself adequately. If you go to the ground, it can be fatal.
Waiting to shoot until you get bit is about the same as waiting for someone to shoot you before you shoot back.
It just dosent make sense.