Carolan:
One of the benefits of enjoying dinner at
California Grill is that, yes, you may return to the restaurant's observation deck to view Wishes, even if your dining reservation was at 5:00 p.m. earlier in the evening. If there is a seat in the lounge or at the bar when you return, you are welcome to order and enjoy a drink before the fireworks.
Keep in mind that the view you will have of Wishes from the Observation Deck is not what you'd expect. From an engineering aspect, it is interesting to see how far back from the castle Disney shoots the fireworks. If you've seen Wishes many times, this viewpoint is like an insider's tour, in my opinion. If this will be your one and only time, I'd recommend viewing the fireworks from the boat launch of the Ticket & Transportation Center or the Polynesian Resort's beach for a frontal view that envelopes the castle.
Wishing y'all a magical vacation!
Whitney