When is the best time to prune Juniper shrubs. Is early spring to late?

Some evergreens are best done in the late fall or winter, others in the summer after the new growth is complete. I am not sure about junipers. Why not call your county extension agent. Every county has a cooperative extension service, even highly urbanized ones. Your tax dollars at work. Since this is a function of the USDA, it is not available outside the USA, although other countries may have similar programs. Help us by giving at least a clue to your location.

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/envirohort/factsheets2/shrubs/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.