Dtiger - that is a good point...typically subject tests are taken on a different date than the regular SAT which requires planning to make sure you get them both in. Most college guidance folks I talked to recommended taking the subject tests in May or June (end of your junior year).
I heard more than one story of students interested in highly academic schools that did not realize until too late that they needed subject tests. Not all academic schools require subject tests but I believe all the Ivy's and MIT require certain ones.