I understand what your system is with the boards Mick. What I don't get is why you, of all people are second guessing people. Why not just knock every door, and let the customer base sort it's self out.
It's like Trippy is saying that he knows who's a good customer because the windows are minging!!
When I've knocked these, some say no thanks, don't want/need a windy. Where as others with minging windows have said they hate cleaning them as it means leaning out etc, and/or no windy had canvassed them, because they were walking their dogs and only knocking on the doors of newly sold houses.
Why complicate matters. Knock and ask. We're both forgetting something though. Some people would rather eat their own left foot, than knock on doors.
On the same note, some people would rather let their windows get an inch of crud on them before they cleaned them themselves.
When in doubt, knock and ask.
Now do you get my point?