Good grades, extensive research experience, clinical exposure, superb references are not enough, good GRE's are often an essential ingredient in making a competitive graduate school application. If you haven't done well on standardized tests in the past, consider enrolling in the Stanley Kaplan or Princeton Review course.
It's more than a little troubling isn't it...psychologists who teach all about reliability and validity place so much weight in this course when there are so many "false negatives"! No! You shouldn't have to take this type of course. But, if I were you, I would!
Students with higher scores are far more likely to obtain interviews, and if accepted, funding! Every year, this question comes up in my summer course, "Research internship in Clinical Psychology". Having spent anywhere between $30,000 to $100,000 for their education, students refuse to fork over the last $800 to take one of these courses. What we are talking about here is spending another 1-2% more on your college education in the name of making a competitive graduate school application.
If your record is as good then with strong board scores, you may be able to gain admission into some of the very best schools in the country. However, if your GRE's are low, you will be denied an interview at any school which uses cut-offs! To my way of thinking, not addressing this possible deficit in an application, is akin to shooting one's self in the foot. Also, regarding the financial considerations, the higher your scores the more likely you are to successfully compete for fellowships. Your scores could make all of the difference. Thus, it could be a very cost-effective decision to take the course in the long-run.
If you do decide to take the course, it is important that you do it at a time when you can devote yourself fully to it. I would strongly discourage anyone from taking it during the semester. Seriously consider taking it in the summer or during a year off.
While I am on this important topic, I should mention that if you do score badly, there are ways around this. I promise to write a column on this important topic. If you do not score well, do not give up hope. There are things that you can do to obtain admission to graduate school. Also, when you become a faculty member, please fight to have the ludicrous amount of weight given to these scores changed.