I've been invited for over 10 interviews - some of them have corresponded purely via e-mail, but there have been some who have corresponded via mail. Or for instance, like UIC - they change the status on their webpage but you need the actual letter for the phone number to call and schedule the interview. My best advice is to cover you bases and religiously check e-mail, school status websites (if they have) and your snail mail to make sure you have all your bases covered. It might sound a little OCD, but it's temporary, and better than missing an invite, right?