Under Michigan law, any use of a firearm is an application of deadly force. Deadly force is legally permissible when it is proportional to the threat. Meaning, that deadly force may be used to prevent death, great bodily harm that could lead to death, or rape. It is entirely possible that an animal could do great bodily harm that could lead to death. So, it is possible that the use of a pistol against an attacking dog would be a justifiable use of force. Keep in mind that the circumstances would have to be such that you are in a place where you have a legal right to be, and that there is sufficient indication that your life is in danger when you fire. If you were to fire when the dog is too far away, or running away, or if you hit something other than the dog, or the dog is a toy poodle, you might face serious legal consequences. The key is that any use of force must be reasonable under the circumstances. I admire your restraint and think that the policy of not using our pistols unless we absolutely have to is the wise course of action.
While the answer contains statements of law and practical advice that are essentially correct, it is worth noting that dogs are covered by different laws than human beings. Under Michigan law, a dog is an item of personal property.
There is a statute in Michigan which states that "Any person...may kill any dog which he sees in the act of pursuing, worrying, or wounding any livestock or poultry or attacking persons, and there shall be no liability on such person in damages or otherwise, for such killing."
However, there is another statute which states that "willfully and maliciously killing or injuring animals" is a felony punishable by up to 4 years in prison.
The practical advice given last week remains unchanged. Only use your pistol if you have to. If you or someone else is in danger of attack by an animal, use your pistol or other firearm to defend yourself and your loved ones.