Sims take care of their own Needs when possible (unless they’re Babies or Toddlers). However, taking over for your Sim is a good way to manage their time, and to lower Needs before they become a problem.
Make sure that your Sim handles any low Needs before they go to work or school, or before they go to bed for the night. Some Needs will be taken care of when they are outside your control, but not all of them!
The key to keeping your Sim’s Needs in the green is time management and, ideally, solving multiple problems at once. For example, having your Sim eat a meal while hanging out with another Sim will increase their Hunger and Social Needs simultaneously. You can also solve a Need while working on a Whim, Aspiration, or Career: Taking a Thoughtful Shower, for instance, will increase Hygiene and cause your Sim to become Inspired, which you might need to complete a new painting.