If you have to turn up a week early for logistical reasons, explain your self and charge them 3/4 of the price, 
Everone should be happy 
i would literally never recommend taking money off a clean. it sets a precedent and some people will start to expect it. you are doing a job for an agreed price
we come 4 weekly. sometimes we will come a week earlier if we need to (sickness, christmas etc) sometimes a week late. people are fine with that. we dont ask permission we just inform them if the clean date is changing
they want a reliable window cleaner, we can do that. but sometimes we need to move things around.
i think if you are getting too much moaning, you are possibly asking their permission too much, or are too desperate to please them maybe.
just tell them what you are doing, and do it. don't ask them. if they want to miss the clean fine.
if they moan or want a discount let them try to find someone else. someone who never needs to move work. lets see how that works out for them!
good customers will be absolutely fine with you moving work a bit