We showed up about 30 minutes before showtime and got front row of the balcony - there was a LOT of seats available, viewing from the balcony was great though as you really got to appreciate the special visual effects - I think the FP was just for when it opened and was far more popular. It's thawed out since.
You can pretty much walk in right before it starts unless it's a busy day. We did this in March. Only the bottom area was full. It is a well done show, but moves fast .