This is highly variable though -- definitely not advice everyone should follow. QBank is helpful in this regard because it has statistics on right and wrong answers, and when you changed answers, if you changed them correctly or incorrectly. From looking at my QBank comprehensive results, I saw that when I changed my answers, I was several times more likely to change an incorrect answer to a correct answer than an correct to an incorrect. So based on my tendencies, it is probably advantageous to change an answer when I see fit. Also, if the OP has a tendency to make silly mistakes, he may want to be fairly willing to change his answers if he feels they may be wrong. On the other hand, people who want to change a lot of answers or who are indecisive may want to exercise more restraint. The appropriateness/successfullness of changing answers is highly individual.