Yeah exactly.. id never use it dry either. So why get customer to sign a waiver?
Not needed in my opinion. If you know how to use a scraper, and use a SHARP fresh blade, with water & detergent you aint going to scratch that glass at all to be honest.
It's not that YOU will scratch the glass but the glass is really dirty, so dirty that any existing scratches will be hidden.
When you clean, you clear away all that dirt/stickers etc which may reveal existing scratches.
Because the customer couldn't see them before (due to the dirt) but can once you've cleaned, a lot of people assume the window cleaner made the scratch(es)
Getting them to sign the waiver, while you explain what I've just put, saves any ballache when you're done.
In fact, I usually say they probably will be scratched.
If they expect the worse, they'll only be more pleased.