Quoted for a builders clean and employed the cleanitup calculation method of thinking of a price, multiplying it by 4 and adding £20.00 and what do you know, I got the job.
I think you've got to price these one-off jobs like you don't want them; high.
But I wouldn't worry too much about the waiver; it may seem like you're being a pain in the ass.
Why don't you just check the glass before you start (a local window cleaner to me had problems with 'pre-scratched glass') and test a discrete area of each pane of tempered glass to see that they're okay before you start?
I don't think scratched glass due to poor manufacturing techniques is a common problem; but I acknowledge it exists.