Truth be told, I still regret I didn't know in was common practice for students who received multiple admission offers to accept them all and then let the "winning" school know by showing up there on the first day of classes. I think things would've been very different if I was aware of the practice and not so naive as to think everyone did as told and made their final decision by the published deadline.
Not saying anyone here should do this, but if there's a legitimate reason why you need more time than the "deadline" allows.... Otherwise, do the right thing and officially decline those you know you aren't going to go to.