British unpredictable weather. last week it rained for 3 days, 2 solidly and one on and off, working in the rain doesn't bother me but ... domestic i guess not?,
Cheers Matt, Leeds. 
Matt,
We've all been in your position. I clean domestic in the rain; last week I did it when it was absolutely hissing down and only had one objection. (By the way, I nearly had a record week as far as window cleaning was concerned; despite the rain).
'What's the point?' the customer asked, 'it's going to rain all week. Can you not come back next month?'.
I told her the point was that all the bird poo, dirt and cobwebs would be removed from her windows - just because it's raining doesn't mean it'll all reapear - and when they dry they'd look clean. Also, she'd still retain a regular window cleaner who stuck to our original agreement.
She reluctantly agreed and tried to pay me. I refused saying I'll collect on Friday; only if she was happy with her windows.
On Friday, she was happy and coughed up.
But, Matt, I remember reading posts like I've just written, when I was short of work and every customer was very precious.
Two years on, I've more work than I can shake a stick at. So I'm prepared to take a customer loss.
Imagine one customer in twenty will drop you for cleaning in the rain (I think the figure is far, far higher).
You'll lose more money in lost income from not cleaning in the rain, than you'll ever lose customer-wise.
Sure, I still feel a little awkward when it's hissing down and I'm cleaning a residential properties windows. But I know I can do a good job - rain or not - so I crack on. If the customer doesn't like it they can find another window cleaner.
That's just my opinion.
If you're sure you can clean a window to a good standard in the rain; prepared to work in the rain; then do it.