A written disclaimer is an excellent idea, but legally, a contract is a contract - either written or "implied" (discussed in front of witnesses).
I've done quite a bit of customer-supplied materials installations. I give them the talk about uniformity and list the problems that may arise and let them make the decision. Our job, after all, is to provide the service that the customer desires.
Just a word of advice - be as polite as possible without taking responsibility for the costumer's errors. A good portion of our job is public relations, and if you're lacking in your ability to persuade, you should avoid jobs like this if at all possible.
Good luck!
