When our doorbell rings and I'm not expecting takeout, I almost never answer the door, because it generally turns out to be either religious folks, political folks, or salespeople. I have a small advantage in that one of the windows in our living room looks directly onto the front porch and I can peek out and see whoever is standing there without them noticing me.
I figure it's easier for the caller to get no answer than for me to have to tell them I'm not interested in whatever has brought them to our door. Sometimes I wonder how successful any of these ventures are; the people hawking Verizon FIOS don't actually work for Verizon, but are subcontractors that add layers of complication to the process, and if I'm going to switch TV and internet providers I'll just go online and deal directly with the company.
Likewise, if I happened to be casting around for a new religion, would I really make that decision based on someone coming to my house? (Well, depending on my spirituality I suppose I might in fact consider it a sign of some sort.) As for political types, if it was a candidate going door to door to meet potential constituents, I might be interested in a conversation, but more typically it's just someone passing out literature that can be left for us to look at later.