If you were lucky enough to get multiple acceptances, it's fine to put down more than one deposit to save a seat while you make up your mind. You won't get your deposit back once the school cashes the check though.
Once you make up your mind, you should notify the schools you don't want to attend. That way they can start giving out acceptances to those on their waitlist (if you were on a waitlist, I'm sure you'd be pretty anxious to hear back)
By May, you definitely want to have notified the schools you don't plan to attend. I am under the impression (based on how I interpreted the wording) that once the schools are notified that someone is holding mutiple spots, they can DROP you (ie - rescind your acceptance) Which is fair because by that time you should have made up your mind and schools need to fill their classes too. At least that's how I think it works.