There's no guarantee that you'll be seated at a table with a view of Wishes. Seeing Wishes on the Poly beach is a lot of fun so I would plan dinner early enough to be finished before the fireworks.
'Ohana meals take about an hour, but as the evening progresses the restaurant tends to run behind schedule. It is better to allow extra time and if you're done early you can walk around and explore the resort. On most days 6:00 will probably be okay, but if you're going at a busy time you might want to think about a little earlier.