I always let my 100 pound dog answer the door with me. He can sense whether it's a wanted person: Purolator/UPS guy, friend of my son, Mailman with a pkg etc; or if it's a not-wanted person: religious tripe, door to door sales, and worst of all, those damn jerks with the 'water heater/gas sales" (For the record, you idiots, we OWN our water heater, and there is no gas in the house). And he lets them know. In no uncertain terms.
My favourite exchange ever was the obnoxious jerk who actually stuck his foot in the door when I went to close it, and said something about me absolutely having to hear him out. Then he TOLD me to put the dog in the back. And I said "No, he LIVES here, YOU'RE the trespasser." I suppose I sounded a little scared/ticked and Miko just let him have it, snarling, baring his teeth, getting his hackles up etc. I glared at the guy and he backed off a bit, though he still had his foot in the door. I said "You're a good boy, Miko...Good dog..." and moved out of the way so the dog could advance to the door and I guess the guy finally got a clue when Miko let out a really low warning growl like I've never heard before. He KNEW this man was unwanted. I slammed the door, and the dog got hugs and pats and treats.
If I can see that I don't know who's at the door, I won't answer it, but this was a young guy (didn't see the damn clipboard til after) so I thought it might be one of my son's friends, Grrrrrr,