Have had that done to me several times.Sometimes they get a drop light out and angle it against the wall, critique minor flaws in the cut-in that can't be noticed unless you're right there next to it, etc.
To answer the Q, i am honest and up front with my customers. Not only do i do everything in writing from start to finish, I verbally go through my process. I point out every flaw b 4 starting and explain how they need to be addressed and at what cost it would it would take to fix. I can't say that i have had problem customers, let them all bring out the spot lights.