Personally I've learned not to keep going back and checking my work, beyond what is reasonable anyway.
Even when I was trad cleaning I'd occasionally be asked to do insides and was amazed that what looked brilliant outside often looked less so inside and was surprised sometimes that customers never said anything.
I now ask my customers to tell me if they are dissatisfied, and that if they do I'll come back and re-clean them. I want them to be happy. A form of guarantee if you like.
I seldom get called back and if I do I don't make a big issue of it, just try to work together with them to get it right. They usually appreciate the effort and if my customers are happy, then so am I.
Like you alan, I like the combo of xtreme brush and black fans