You should train your core in all three planes of motion. If you don’t, you’re probably overworking certain muscle fibers and underutilizing others, leaving you more prone to injury and less likely to develop those ripped abs that you want. Therefore, to hit all the planes of motion, you need to “curl it”, “twist it” and “bend it” when doing your abdominal exercises. You can do very simple exercises to hit all those planes of motion. “Curl it” with a weighted bench crunch, “twist it” with a hanging knee twist and “bend it” with a side plank.