I have all ready trimmed my boxwood lightly. You can trim any time. A good time is when the boxwood has put on new growth and is feeding itself, so to speak. All so . observe your boxwood carefully for the moth that lays the egg that turns into a big green worm that eats your plant. That is why most stores that sell boxwoods have a no return policy. Southern Ohio, and southern Michigan are under advisement.
If you want to trim your boxwood to look like mickey mouse, now is a good time to begin. Just kidding, but some do it.
Yes , you can trim back by one third. I like to open up the plant so it gets more light and pushes new growth.