I never approach a knock at the door without my handgun. And I never open the door until I know who is knocking. I also don't stand in front of the door when making my inquiry as well. And if it should turn out to be an unknown person, I further inquire and request they state their business. Once all of those initial steps have been taken, I position myself in such a way that doesn't put me in a vulnerable position and gives me the advantage, distancing myself so that I'm not at risk of being rushed and relieved of my weapon, or gunned down in my door way. And if the prevailing circumstances don't require that I open the door, I answer their questions, bid them a good day / night, and then observe them from a safe vantage point to ensure they have left the property.
And I think one of the most common mistakes made by those who think, just because they have a weapon all is good, is to keep family members out of the picture, so as to not become hostages that would ultimately force the thought to be prepared citizen into surrendering their weapon. In most homes, children will often rush the door when Mom or Dad approach the door with the above routine. In my home, it has always been a rule embedded into all others to stay out of the way, don't alert the door knocker as to who, or how many are in the home, or where they may be positioned by the sound of their voices.
GS