<p>SAT IIs seem pretty much essential for the top tier of schools (especially Ivies), so it would be in your best interest to take them.
The advantage to the subject tests, as well, is that you can take them in areas where you are strongest. So, for example, if you love chem, take the chem test, and show the admissions committees that you’re good at it by getting a high score.
It can only help–because if you happen to do poorly, you can opt to not send them (unless the school requires them, of course).
Best of luck!</p>