Arcas,
In my part of the world, when we winterize, we will open the ball valves halfway to keep water from being captured in the side of the ball valve, thus freezing and cracking the ball valves. This is only possible if you have a primary point of shutoff upstream of the backflow preventer. Likewise, if your backflow preventer is located in your basement, it may not be necessary to have the valves halfway open.

All Wet
Texas Licensed Irrigator #1897 since 1984