Just because their plans say that, you still have to have utility locates prior to digging anything to comply with the law. And if there is ever any doubt about depth, potholing can take place to find the utility before digging.
That being said, utility companies have some of the worst maps ever. General location and approximate depths are about all they ever have because so little of that ever gets as-built information. Half the time, you're lucky if it's even shown on the right property. Designers do their best to get locates during design, but that process is notoriously a crapshoot.