Visiting can only be a positive, but I'd sincerely doubt that it could be a negative.
I was waitlisted and was able to meet with the Dean of Admissions and explain the problems with my GPA and some other question marks on my application. He was generally responsive and said that he would relay the message to his admissions committee. I was one of the first people off the waitlist and I have to believe that it had something to do with that meeting.
Just a personal experience, but if you aren't accepted yet, it could probably only help to make the visit, unless you do something really stupid.