Not being funny but why should any customer make allowances for any service that they are paying for?
Would advise you, that if the windows are really bad when you go to quote ask the customer when they were last done and what are their expectations, based on the answer of how long since they were done. Take your time on the quote, even do a small area as an example to yourself of what the finish will be like. If you don't hit the customers expectations on the first clean, unlikely you will get follow ups.