As we're not around when the windows dry, from time to time I spot check with the odd customer that the windows are comming up fine and I always make them feel comfortable with making a genuine complaint if necassary. It never fails to suprise me though how many don't check and are happy to just see you regularly or know you've been. Others though are quite finnicky and can be a good guide as to how well the system and your preferred method is working. If rinsing brush on doesn't work for them I don't do it.
I've lost a total of four customers through switchng to wfp (top only for the most part as I still enjoy working with a blade). Those I lost were predjusticed against it from the start.
However, I've gained a lot more new work since going WFP, some of which is as a direct result from making the switch.