It would be nice to inform them.
Most have some sort of online acceptance/sir web site - just log on, click a few times, and you have officially notified the school. It works really well, I have done it 3 times this spring, lol.
Of course it is polite to let them know you will not be attending. Most schools send you a postcard or something similar in your acceptance package, where you check off whether you are attending or not, and some ask you why not/ or where you have decided to attend. You don't have to give them details.
Anyway, if they don't receive your deposit by their deadline they will assume that you are not coming. I doubt that most schools notify their waitlisted candidates until June at the earliest?