Unfortunately we don't have the climate to be able to say "its definitely not going to rain for the next 8 months - would you like your windows cleaned?"
People have to realise it RAINS alot here.
If they require a reliable and regular window cleaner that is what you need to allow for - the rain, of course not everyone works in the rain, i only work in light rain/drizzle nothing heavier.
Most customers i have i say don't mind maybe they probably do mind, but they know its going to rain some time or another so will accept it.
I must admit, i had one of these calls a few saturdays ago - I got, your work is great, but what with all this rain weve HAD... (yes the main reason to get your windows cleaned i thought)... and the poor weather expected... (right so i'll contact you when the summer comes?? i thought)... id like to postpone for a better day (it wasn't raining, it had rained through the night though).. :

Ive not phoned them since, and when its next due - if i get the same reply, come mid december - then i will say that im unable to provide them with a regular and reliable window cleaning service so, should they require the services then they can call me, and pay the "one off" rates, rather than the regular rates.
As using WFP im confident of a good drying result should it rain after ive been and dries within 24hours of the clean, so i offer my rain guarantee - never had to go back yet - the only time i clean my own windows is when its raining!!