<p>Some do accept the ACT with writing instead of SAT I and II (for example, Yale, Vassar) and some don't (for example, Harvard, Georgetown). </p>
<p>If they are required, then yes, you will be looked down on if you do not submit them. Your application will be considered incomplete. If they're not required, but only recommended, then submitting the ACT (with writing) alone should be sufficient.</p>
<p>Read the websites of the schools you want to apply to. They don't hide their requirements.</p>
<p>I agree with Chedva that if SAT IIs are only recommended, that sending the ACT alone is sufficient. However, if you are applying to very selective schools, I'd recommend taking SAT IIs and sending them too. JMO</p>
<p>Sometimes SATII can be enough to get you out of a class requirement- I don't have to take as much math for my major because of my Math I score (which wasn't even very good). I'd rather take an hour long test than a semester long class in calculus.</p>