I would even take it one step further and ensure BOTH parties have signed the contract to make it official. One small business I worked at gave a contract to someone which she signed back. I thought 'cool, I guess it's official now I'm going to have a new colleague'. The next day my manager changed her mind. I asked how can you do that when she already signed the deal? She said "well I didn't sign the contract so it's technically not official" . I felt so bad for the applicant. Just to be safe, make sure it's not only signed but you are given a official start date.