Do you get freezing there? If so your minimum should be at 24" to keep from freezing, weither you winterize the system or not.If you don't get freezing then 12 to 18 inches will be sufficient enough.I hope that this helps.
If what you are calling "main lines" is the pipe that proceedes the valves (i.e. lines under constant pressure), from what I've been told, the uniform plumbing code requires PVC main lines to be at least 18" underground.
If what you are calling "main lines" is just the primary distribution line that comes AFTER the valves, then it's back to what your weather is like.
Here in Northern Michigan we install the lines to a standard depth of about 12" all around. With proper winterizing, this is sufficient. All the local contractors in this area install between 8-16".