Just take the placement exams. It will make your Preview experience a million times easier. There's no reason NOT to take them, other than pure procrastination. If you're going into the medical field, then it's definitely a good idea to take them. Otherwise, it's going to take you that much longer to complete your Pre-Med requirements.
Now, if you've taken AP/IB exams already for math or chemistry, then you may be able to get out of the placement exams. However, I suggest you take them anyway, just in case.