Depends how busy the store you're going to is, but I wouldn't go in expecting to be seen. I had an appointment yesterday and although the Genius Bar didn't seem that full, three people got turned away and were told to make an appointment (earliest available was next Monday) and were told they couldn't be seen even if they waited.
You could try phoning the store, but it's a hundred times easier just having an appointment and means you will definitely be seen.