I try not to disagree with people on here, so I am not saying to ignore mousehockey, he does have a valid point. But I can tell you that we do this every time we go and at least for us it works out. We reserve for 7:30 PM, we arrive on time, there is usually a 15-30 min wait for your table, and are seated around 8.
Sometimes we are lucky and are on the patio, sometimes not. But if you tell your server when you arrive, they will try to time your order so you do not have any hot food arriving at fireworks time, and will walk you out to the patio and let you stand there and watch the show. Then return to your table and finish your meal. Once our server didn't get the timing quite right, so we walked out just as our meal arrived, and figured we'd eat cold food. When we walked back in, he had placed silver covers on them to keep them warm for us.
So our experiences have worked out well.
As a matter of fact, the last time we ate there in April 2017, as we were finishing our entrée, the gentleman from the check in desk came and asked our boys if they wanted to start the fireworks. He gave them crowns, sashes of Mickey stickers and a magic wand. He counted perfectly backwards for them and when they waved the magic wand.............the first firework went off. They were beside themselves! Really believed they did it........there is still a little magic at WDW.