Always, always ask why.
I've had one customer saying she was cancelling because of poor quality work, that had her BBQ out next to the windows and my colleague hadn't spotted the grease marks on the glass before cleaning. Once wet it's very difficult to spot, Same with lime scale stains due to splashing of swimming pools or over zealous use of the garden hose on plants.
Very easily rectified situations, but if I hadn't asked, I'd have lost 2 good paying customers due to lack of attention before commencing cleaning.