I think it's one of the most important things to do once you've been accepted to the school. (Wait until you hear from all of them, and then make a visiting circuit with someone whose opinion you trust and who is seeing the school(s) for the first time.) I did this with several schools and it made a huge difference for me; you get to talk to random students you see, you can get "behind the scenes" more and see things that weren't specifically set up for the interview. Also, you can browse the campus and facilities more and you don't have to feel rushed. Hey, you're already accepted!
I wouldn't do it before you're invited for an interview; during one, they're really good at spending the day introducing you to the school (if you cannot make another visit once you're accepted.) Also, you don't want to show up and look unprepared or seem obsessive...if they want you for an interview, they'll let you know! Good luck!