I have always just put a disclaimer in my proposal that non-traceable plastic water lines, irrigation lines, and or sewer lines (which for the most part are not a problem here because of the frost depth) I tell the home owner they have the option to have the people who installed them to come out and mark them if they can, but if I can't locate them with my RDF, repairs will be up to the home owner.
like I said most water lines and sewer lines are at or below 4' here, but irrigation lines are almost always un-traceable (except copper ones like my house) and using MS word for my proposals, I adjust each disclaimer to each job I'm doing, I have never had a customer yet try to make me repair or make me pay for a repair for hitting their irrigation lines, now I have hit a couple gas lines, one with a ground rod, and one with trencher, and we had to pay for one of the repairs, the other was wrongly located by the locating company, but I get locates done on every trenching job I do, and then break out the RDF and do my own locates on everything else.
I at one time shut down half the cities power on a dig with my back hoe because of wrong locates done by the locating company for all the utilities, they had marked on the driveways on both sides of the property that there was no utilities with big arrows pointing to the property, on the neighbors concrete drives in paint, I hit a 7200 volt primary and the 240/120 volt secondaries coming from a transformer across the property, just missing a 2" gas main Scraped it but didn't break it, the utility took us to court and because I had taken photo's of all the locates and damage, we won hands down, not only did the locating company had to pay for the repairs, but they had to pay to remove the paint markings on the driveways of the neighbors that should have never been marked in such a way.