Older systems here were done in PVC and there is still the occasional one. Most systems here are now poly, and most companies will use poly for a mainline, if they do a mainline. And they rarely use a MV, and rarely is there a problem. The only time there is an issue is when the pressure regulator fails in the house, and pressure then spikes to 125-150. Then we will see the occasional blow out in EITHER poly or PVC if the main was done in Class 200. Not all pipes will crack from freezing. I have many customers that do not blow out their systems, and some have damage and some do not. When winterizing a system, my main concern is backflow preventer, manifolds, mainlines, and valves. Or if the system is all PVC or copper (yes we have some older systems that are all copper). The occasional split poly pipe or head is no big deal. It's the 20'-60' of shattered PVC that chaps my azz.