contacting the office of minority affairs/multicultural affairs (or whatever else they may go by) can't be a bad thing. if nothing else, it will allow you to start practicing your "why i want to come to your school" routine. it may also help you to figure out why you want to go there, or why you may want to stay away.
at best, it may help you to gain an advocate when your application comes up for review at committee. if the office gets to know you, perhaps they can steer you towards what to say or not to say on your application. perhaps they can help arrange students for you to meet during your interview, or a place to stay when you interview. at some schools, there may be a dean that is in and/or runs the office of minority affairs. having him/her familiar with your name when your file comes up in front of all the deans when deciding whether to admit or deny a candidate can be a good thing.
so, yes, you need to contact the office of minority affairs for the school's you're applying to.