Hi Jessica,
Great job in snagging a dinner reservation at Be Our Guest Restaurant. That's one of the hottest tickets in town these days. (Rightfully so because it's pretty amazing!)
Now, on to your question!
If I were in your shoes, I'd plan to snag a reservation at 'Ohana about an hour before that night's presentation of the Wishes Nighttime Spectacular. As the evening hours progress, it's been my personal experience that the schedule at 'Ohana gets a little backed-up, so you'd want to have enough cushion to get seated at your table, relax with a tasty beverage (my go-to drink at 'Ohana is the Kona Longboard Island Lager), and have the first course come around so you can prime the tank before the fireworks.
That would be my approach if you were hoping to watch the fireworks from inside of 'Ohana (which is very fun!). If you'd rather watch from the beach at Disney's Polynesian Resort, then I'd plan to get a reservation about two hours before that night's presentation of Wishes. That way, you can deal with any potential delays in getting seated, you can take your time eating, then you can scope out the perfect spot on the beach.
Here's a little tip from your buddy Derek:
While it may be hard to resist the generous offerings of steak, shrimp, chicken, and pork that's brought around on skewers by your "cousins," make sure you save a little room for dessert. The evening will be capped off with a huge portion of bread pudding served with vanilla ice cream and a banana/caramel sauce. I fancy myself as a bread pudding aficionado, and this example has to be one of the best I've ever had anywhere.
I hope that helps! If you should have any other questions, please feel free to write back and I'll be happy to help in any way that I can.
Best regards,
"Cousin" Derek