The print outs are considered your certificate to the customer. They should have a place for date, parameters, and technician on each sheet. They also have the NVP listed, which if your client wishes to check your honesty, can do using that reading.
I won't do an install without testing for reasons cited by nforeman, however I don't break it out on a bid so the customer has no choice. I do tout the fact that all jobs we do are certified and carry at least a 5 year warranty. The only choice I give a client is if they want us to do the work, or if they wish to upgrade to a 15 year warranty, most don’t. I never let a client lower my standards.
Truth be known, the average life of a cabling installation is less than 5 years, not because of faults, but because it is usually upgraded or the client moves and the system is reconfigured by the new tenant.