You don’t have to. Your score is good for 3 years. I applied for my first time on last year’s bid, took the AT-SA and got referred but not hired. I’m planning to take it again for this bid since I now have a better idea of what the test is like. If you got referred on the last bid and were “Best Qualified” or “Well Qualified”, I personally wouldn’t retake the test and risk getting a lower score.