If you still have full motion without pain or other symptoms, you are probably safe to begin tennis. There is a risk of future injury, but there is no way to predict this. Warming up before exercise or activity (including tennis) is always a good idea. Spend about five or 10 minutes stretching the muscles of both shoulders, and move your arms through various tennis motions gently and slowly.
Your therapist can retest the strength of your shoulder and prescribe a specific warm-up and exercise program for you. This may be a good way to prevent future injuries while still enjoying the fun of tennis.