You should take the SAT at least once junior year so you're not relying completely on your scores from one senior year test date. Taking the test this year will give you more information about your strengths and weaknesses and lead to more productive studying over the summer. If you wait until senior fall to take the SAT, you'll face way too much pressure. It will be your first time in the official test-taking environment, so you can't be sure whether you'll thrive or crumble under the stress. If you end up having a bad day, you'll have no choice but to send that score to colleges.
Taking the SAT junior year is a win-win situation because you might do so well that you don't even need to take it as a senior, and if not you will at least have more experience under your belt before your final attempt. Also, if you're already in tough classes and are a solid student, you may be able to get a pretty good score without devoting much time to studying. Just take a practice test the week before so that you're familiar with the format.