Someone used that method "on my patch" some time ago.
They got very few customers,
1 because they didnt like the idea of people on their property (the only way to know how many window as h20 says)
2 they like contact with the person who will be on their property, you could be anyone.
As a side issue to th ones who tried it near me. One of my customers who accepted the quote, (he was cheaper), was broken into by a man, during the night, while the whole family were asleep upstairs!!!
As it transpired, the burgler was a neighbour of the guy who had gone around pricing up the work. It was never proved the two were connected, but it goes to show, if we dont have contact with our customers, initially at least, we appear very shady.
Be warned.