I'm currently a 2014 cadet and, though the instructions say otherwise, I was told that I should have mailed my form in.
After Beast, the plebes in my company all talked about getting paid for July (the first month of Beast), yet I had nothing in my account. So I went down to the finance office and they told me that I was supposed to mail AND bring my Direct Deposit form on R-Day (strangely, there was no station for turning in direct deposit forms, so I,as well as about two other New Cadets I can recall had to hold on to our forms during Beast). Ultimately, they paid me for the month in cash, but I think it was a lesson learned.
So my recommendation would be to mail the Direct Deposit form to the finance office AND bring a copy on R-Day, just to be safe.