On WDWcollegeprogram.com the login area says that they don't want you to log in until you receive a letter. My question is why. Does it interfere with something when you login or what?
Yes... it will interfere with the entire processing bit. They make it clear not to do so until you receive a letter. DON'T DO IT... as it might take your letter twice as long to get to you, providing it even comes after you log in.
