It really depends on what kind of viewing experience you would like. I personally prefer going for the second showing (although I can't recall if there is more than one show this week) so that I don't have to wait for hours. For the first showing, people usually start to line up as early as possible, at least 2 hours in advance, sometimes more. If you want to be right at the railing, you're going to have to get there early, but I recommend being a little ways back, because you can see everything better.
Do you want to sit or stand? For sitting, you're going to have to get there pretty early. It is possible to play cards or have someone get snacks and such, but as time gets closer, people might try to snag the empty spot, and it gets harder to work your way back through the crowd in order to get to your spot, but it's not anything you wouldn't be able to handle.
If you want to stand, you wouldn't have to get there as early, but you'd still want to make sure you are the first people standing behind the "sitting people." Your sight line is unobstructed. It is possible to jump in the back of the standing area and still have a good spot, as I have done that many times, from 20 minutes to showtime to a second before the lights turn off and the music starts. It really depends on your preference.
The park is going to be pretty busy, and the area where you would be viewing is going to be stroller overflow parking during the day, so that might impact how early people start finding their spots. Someone else might be able to help with this. I'm pretty sure I saw someone already set up yesterday maybe as early as 5.
As for viewing spots, I prefer to go right in front of POTC. It's not exactly the middle, just off to the side, but there are no trees and very few lightposts and such to get in the way.
Hopefully my novel answered your questions!
