Just be sure to keep your receipt. They sometimes ask to see it when you head back up. Last time I arrived 20 minutes early, and I got back up without a problem.
We do this all the time. We always make an ADR for 5:30pm (earliest seating time available) then after dinner we leave and go around the monorail loop to shop and look around then we head back to Cali Grill and go back up to watch Wishes out on the balcony. Like one PP said, make sure you have your receipt, however, every time we have done it we had our receipt but they just waved us on without really looking at it. Have a great time, the view from up there is awesome.