Make sure your T&C's exclude staining like paint, cement, water proofing spray etc.. You should only be cleaning dirt, air bourne stuff like tree sap, birds muck, pollon, flies etc.
If they want things like paint/cement removed, then you will need to use a scraper - as such it can be possible to scratch the window.. so ensure you get them to sign a scratch damage waiver!
I dont touch any windows with a scraper or other construction removal equipment/products without getting the customer to sign a waiver.