Matt,
Unless you've a strong feeling that these are problem customers, do them one more time and stress to the customer that they should phone you straight away if the windows aren't clean.
Tell them if there's any problems you'll be round straight away to sort them out; with your ladders if need be; but also reasert that with 99.9% of your work, the windows and frames dry leaving the windows clean. Just make sure they have your contact details, so there's no lack of communication.
Some customers just don't like the pole; I lost one two weeks ago to a trad guy and that customer sacked her previous-to-me window cleaner because he used a pole; I must've been cleaning her windows for about four years.
Another, when I first started with the pole, a customer complained her windows were spotty, so I went and checked the following day and they were gleaming; the only marks were on the INSIDE of the window; her previous window cleaner had used a WFP; and she just assumed they would be rubbish.
Window cleaning would be much better if we could run some sort of 'sensibility test' on our customers prior to taking them on.