Bargaining is important in some situations. For example, when shopping for crafts in souvenir stores or when you visit a market (except for food), they expect you to bargain. With taxi drivers, if they don´t use their taxi meter (which can be the case at night), you should negotiate the price. It´s not common to bargain in restaurants. In large shopping malls or chains, they will not bargain.
It will be an advantage if you can speak or at least understand Spanish. Always be realistic with your offer and don´t be aggressive. Keep a light heart and a smile when bargaining. Try to remember that small amounts can mean a lot more to your vendor than to you.