This is entirely up to you! Some people can't imagine meatballs without onions and they dice or mince them in with other ingredients. Other people don't like biting into them when they bite into a meatball so they skip it. However, onions do add a certain sweet flavor to most dishes. So if you want the flavor without the bite, use onion powder, or add the onions to the sauce instead of adding them to the meatballs.