For most recipes you do not need to stir while cooking. In fact opening the lid to stir decreases the heat inside your slow cooker and thus increases the cooking time. However there are a few recipes that do call for opening and stirring and thus, they have been adjusted for the additional cooking time. So, unless your recipe calls for stirring, don't. And when you do have a recipe that calls for stirring while cooking, open the lid, give it a quick stir and then close the lid quickly to prevent too much heat loss.