I deal with this issue right from the start. Every time I quote, part of my 'patter' is 'We work in most conditions, including rain, but all our work is guaranteed. If there's a problem, pick up the phone and call us and we'll come back.' I don't bother with things like 'within 24/48 hours' or any other conditions, I offer an unconditional guarantee. We never get called back, but telling the custies about the guarantee helps to create a feeling of confidence in our service.
The guarantee is printed on the back of my quote forms, and the quote package includes a separate sheet with the guarantee on it, and as a precaution if we turn up to start working in the rain, we have leaflets ready that say words to the effect of 'Today I have cleaned your windows while it was raining, I have to do this as window cleaning is my only source of income, but as you know I offer you the guarantee that if there is any problem you only have to phone and I will return and rectify it'.
This leaflet goes through the door before work starts, so if the custy is on the way to tell us not to do it, they see the leaflet first. It takes a brave custy to still go ahead and tell us not to do it when all their objections have been dealt with before they say them.