You are encouraged to take advantage of IntroSems, both for WAYS Requirements and for general interest. IntroSems are an excellent opportunity to interact in a small class with some of the university's most prestigious and talented professors. They are also a great way to really get to know a discipline prior to completing the preliminaries for upper division work.
You should consider signing up for IntroSems in areas that are new, but intriguing to you, as well as in areas in which you already think you might major. You might find a new passion or, perhaps even more usefully, discover that you are not quite as interested in a field as you thought you were before spending two years taking prerequisites. You might also reaffirm your interest in a field and possibly find a recommendation letter writer or even a faculty mentor. An IntroSem is truly an unparalleled opportunity. If you can find a way to take an IntroSem every quarter, you will find yourself well rewarded. At the very least, every student should endeavor to take at least one Introductory Seminar during his or her time at Stanford.
See the other perspective--some faculty have shared why they teach IntroSems with us.