I have never, nor will I probably ever, walk away from a detail job. Reason being -pre-inspection. Before ANYTHING touches the car, I walk the vehicle with the owner and explain to him what can be done with the package he/she is willing to pay. If they accept, its in writing. If they want better, I will gladly adjust my rate to accomodate. Plain and simple. Plus, my pricing sheet clearly states that the quotes are for "average condition or better", and subject to change upon walk-thru. That clarifies any and all expectations of what I can provide for the customer, as well as sets their expectations on what the end result will look like.
Now, if I get into a detail and things aren't working out the way I AND the customer expect, I will take the hit in the piggybank to make it right. There have been too many jobs to count in which I had to talk to the customer and say "hey, this vehicle isn't in good enough starting condition for the originally quoted price". You know what? I still landed 100% of the jobs, and 80% of the time got a higher fee to make it what the customer expected, and still made a profit.
Lets all be honest -there is no such thing as a "perfect" detail. If there is, I haven't seen it. There are a lot of amazing detailers that have done unbelievable jobs, but I doubt anyone can come up with a "perfect" detail, or the perfect car in which to take as a job. That's the challenge of detailing -to bring out the best condition of a particular vehicle in which you are working on for the money you are being paid. But I certainly would never turn a customer away, especially after anything with my name on it touched their car.