Is this privot, you are talking about?
When you say early do you mean early spring or when they are little plants before they get big?
Oh, I have box hedges too, not just privot.
A more knowledgeable person than me, said to trim it only twice in the summer. Make sure the first time is up in the summer past frost and the next should be a couple of months before frost. Watching my box wood knot hedge I can see why. A little growth tips in the winter keeps the rest of the plant from getting damaged.