When pruning your hedges and shrubs, be sure always to aim for a pyramidal shape -- wider at the bottom and narrower at the top. This is how hedges grow naturally, and it's for good reason: This way, sunlight, which is necessary for photosynthesis (which, essentially, keeps plants alive) can reach every part of the plant. If shrubs are pruned in such a way that the bottom ends up thinner than the top, the top branches will block sunlight to the bottom of the plant. That could result in dead branches at the base of the tree.
And, according to Peter Felix, a certified arborist with Tree Health Management in Farmingdale, a pyramid shape "also makes plants less susceptible to snow damage and wind throw."
Plus, if you thin the plant out at the bottom, the only way to correct that would be by radical rejuvanation -- cutting the entire hedge down to 6 to 8 inches from the ground and waiting for it to grow back. You don't want to go there.