You don't really need a disclaimer as such. What you need is a report of faults to be noted during the pre-clean inspection. I will write on my survey form something like "Pre-cleaning inspection has revealed bleached staining to lounge carpet......." I will then request the householder to acknowledge this with their signiture.
If you are to carry out any"dangerous" procedures that could potentially damage the item(s), then include this and the risks involved, permission given without liability on your part, on the survey form and then a signiture.
This is not a get out clause for poor workmanship eg cellulosic browning from poor drying or incorrect chemistry.
Safe and happy cleaning

The Ken