My experience is as per Paul Coleman. Its happens - don't let it eat you up.
If you are asked to quote a house with dirty windows, the chances are that they have lived that way and will continue to do so. You are being used to clean the windows for a special occassion - visitors etc. When they tell you they have been looking for a window cleaner for ages, take that as a warning. Messers will always be messers. They stay the same - its the window cleaner that changes.
Charge extra for the first clean with a reduced maintenance clean. If they change the terms of the original agreement, you have lost very little.
I don't argue anymore, I just leave them thinking that I will do them as and when. I still will do them as a one off clean in the future if they phone, but at a premium price. TBH I think I have had only one person ring back in 9 years.
As Paul says, that's one less person to deal with. His is the voice of experience.