HI,
Pruning juniper shrubs, according to the article listed below, is best in early spring, after the first signs of new growth.
http://www.ext.vt.edu/departments/en.../feb88pr2.html
The site above explains why.
Evergreens basically are very dormant late in fall and in the winter. I have 6 evergreens in front of my house in VA, and I use electric hedge clippers on them when they start getting too big.
I usually clip them during the late spring or summer, when they are full of new growth and the sap is really flowing. I do wish you the best, and happy clipping.