If your tree is in a pot, it isn't the top growth you need to worry about (although there should be a balance between roots and top growth). To maintain the plant in a pot, you will need to root prune. Something I have never done, but I did find this:
As to pruning top growth, when the tree is dormant is best. Pruning during the growth season just encourages growth, which is usually weaker and less able to stand up to weather conditions. When cutting off branches, cut in the direction you want the new growth to go--not inward to the center of the plant, and not resulting in an umbrella effect.