There is no Firework SuperThread like F! and WoC SuperThread.
SB Castle is also part of the show with fireworks launching from the castle, colorful bright lights illuminate the castle, and projections on the castle. Also don't forget that you can watch Tink (RDCT & Magical) and Dumbo (during Magical) in a great spot during the fireworks.
The best spot to watch the fireworks is in front of SB Castle. A lot of people have different personal preference of where they like to watch the fireworks in front of SB Castle. The very closest spot to watch the fireworks in front of SB Castle is the curb sidewalk. The curb walkway closest to the castle will be roped off for people that are planning to walk to another land during the fireworks. So you will be on the black road when you watch the fireworks.
Mostly you have to stand up during the fireworks. The CMs will make an announcement to the guests to stand up for the firework show about 10-15 minutes until it starts. The only place where you can sit down and watch the fireworks is the benches around MS. There are benches around the Hub which you can use to sit down and watch the fireworks. But since everyone is standing, you will almost likely have to stand up too if you don't want to be blocked out when you are sitting and watching the fireworks.
For what time you should get there depends on how important the view is to you and your group. I have seen people wait more than 7+ hours for a spot in front of SB Castle for the fireworks (but that is for New Years Eve and 4th of July). But on average, I have seen people claim a spot about 3+ hours. That is most likely for the benches in front of SB Castle. You can still get a spot on the road half hour prior to the show. But since you are doing it during the summertime, it can fill up in one hour. I have seen people force their way in when the fireworks start.
You can stake spots around the Hub in front of SB Castle for the fireworks. But mostly they are on the curb side closest to the castle and the curb side of the Hub. Mostly I have seen people set up around 2 to 3 hours prior to the fireworks. You can bring a blanket and use it to claim your spot for the fireworks. Just like F!, where you can bring a blanket and claim a spot in the sitting area right next to the ROA. You just at least need one member of your party to watch the blanket while the others can go on rides, get food, bathroom etc. and still have your spot.
Just like what HydroGuy said that the fireworks can be canceled due to winds and lightning.
For me I always get there 2 to 3 hours prior to the fireworks right on the curb side closest to the Hub. This is mostly for videotaping the fireworks. I always like to videotape the fireworks and this is where I videotape it from the pictures below.
Magical Fireworks
Here is my video of Magical that I have videotaped right next to the Hub.
Magical Fireworks at Disneyland 5/27/11