Whether you need to take a prep course depends on your own testing personality.
If you:
- tend to do well on standardized tests
- feel you have a good understanding of
the material covered in your courses
- have a lot of discipline for self-study,
- are well organized
then you shouldn't waste the money.
However, if you
- are not a naturally good test-taker
- feel there are gaps in your knowledge
- do not have the discipline for a course of study that will last 2-4 months, or
- are not well organized,
then a prep course will give direction to your study and help you focus on the important things. You will still need to put in your own effort - the course won't do it for you - but it will provide an excellent base.
Evaluate yourself honestly to arrive at the choice that's right for you!
Best of luck.
p.s. here are my credentials - four years teaching SAT prep, three years tutoring premeds in chemistry and calc, SAT and MCAT scores above the 95th percentile, currently enrolled in a top-10 med school - just to give myself a modicum of credibility