Unfortunately, This is a question that is overlooked at the time of material installation more often than not. The short answer is, If you live ANYWHERE that there is snow fall and have any exposed roof areas that could pose a hazard if snow and ice were to come crashing down than, yes, you do need snow guards.
The emergence of synthetic materials such as DaVinci as a popular option for both re-roof and new projects alike has changed the dynamics of roofs across North America. The synthetic materials rival metal in slippery-ness. Due to their nature, which also ensures the longevity and colorfast beauty that drove you to the purchase of a synthetic material to begin with, their is no porous or granulated surfaces on the material that allow the ice and snow to "grip" the roof.
When it comes to snow and ice, your roof is going to perform in a completely different way than it did before you installed the synthetic product. Avalanches and snow sliding off will occur on ANY pitch up to about a 16/12 (at that pitch the roof is almost vertical and while snow will slide anyways but not accumulate in a manner that could pose a threat).
It is important to discuss snow retention options with your contractor and make sure they include it in your initial quote. If not, you will be looking into retro-fit installation options after the first couple snow falls of the year. We here at Rocky Mountain Snow Guards are more than happy to help you and your contractor with the design and layout of a beautiful snow retention system that will not detract from the beauty of your synthetic roof. Please feel free to contact us for any additional information or to request your free quote.