Actually no, it is not a limitation to fly without the yaw damp single engine. Bad info. While you do need to properly trim out the airplane manually first, once it is properly trimmed you may engage the YD. Personal technique/preference as to whether you do or don't. Most of my instructors taught to engage it. I did, and it will definitely help mitigate the Dutch roll tendency, IF you trim properly first, and avoid large, rapid changes in thrust.