I clip all flight feathers on both wings. Clipping just one side can result in injury from off-balance flying. Clipping both sides takes away their "lift" but doesn't affect their "aim". I've seen chickens wth busted heads from accidentally flying into a building from uneven clipping. Just be sure to look at the feather shaft before clipping. If you can see a blood supply running up the shaft, don't clip it. Otherwise, it's really quick and easy, and doesn't hurt the bird a bit.