Will colleges hold it against me if I take the ACT instead of the SAT and live in the East Coast?

As long as the schools require either the ACT or SAT on their website and specifically say so without indicating any sort of bias toward one or the other, there's really not much to worry about. More commonly, I see schools in the Midwest saying they only take the ACT, as opposed to other schools saying they only take the SAT. The tests are officially regarded equally, although this rumor is well-known around the country. It's a broad generalization that the applicants to schools on the East and West Coasts send in their SAT scores more commonly than they send in their ACT scores, but that doesn't mean that you can't get into a school if you only send your ACT score, especially if you're not aiming for the top-notch colleges where college admissions is so competitive that perfect scores on either test can't guarantee you anything. I know for example that I sent in only my ACT score (33), but got into a top 5 liberal arts college that based upon this rumor, would have preferred the SAT.

They aren't 'supposed to' - many schools specifically prefer one or the other. It's true that SAT is far more common on the east coast, but many schools still accept ACT. since it varies by school, you'll need to look up the info for the colleges you are interested in.