Let me state an obvious fact: if you take the IELTS three times, you will get three different scores. This is due to the way you feel on test day, the level of preparedness you have, and, despite IELTS’s claims to the contrary, some tests WILL be easier for you than others.
Since so much depends on your score, you should maximize your chances of success. In order to maximize the likelihood of success, you’ve got to prepare in advance. This means taking official practice tests and spending time learning the information and test taking strategies you will need to succeed.
You can always retake the test more than once, but remember that you will have to wait a minimum of three months before retaking the test. Don’t get into a situation where you need a higher score and can’t afford to wait, so don’t take the IELTS as a “practice” test.
Feel free to take sample tests on your own, but when you go to take the IELTS, be prepared, be focused, and do your best the first time!