Do you care about what seating you get? (Where in the room you will be?) If you do care about seating, then arriving as early as you feel is possible for you will be more beneficial than arriving later. I'm not sure about
D23, but other conventions do have separate lines for reservations and for standby. For those wanting to do stand-by, and you REALLY want to get in, then arriving as early as they allow the queue to form is best. That said, I have seen panels that didn't even open the standby line at all, because the reservations completely filled the room.
I appreciate this stuff about lines might be hard to hear considering the cost and time it takes for a large-media-convention like this. It's why I tend to prefer fan run cons, but usually go to giant media cons because I'm either doing booth duty and/or seeing friends that I only get to see when they travel for these big media cons.