The attraction lines are at their very shortest during the early presentation of Fantasmic. Also, the Fantasmic stadium is much less crowded for the 2nd performance.
We like to hit the headliners again while the bulk of the crowd is at the early show. For example, last summer we visited Toy Story at about 9 pm. The posted wait time was 70 minutes, but the actual wait was closer to 15. We were able to ride 4 times in about an hour and still had plenty of time to make it to the 2nd Fantasmic after the park closed. The amphitheater was no more than half full.
Another night we let the kids ride Tower of Terror 13 times with literally no waiting, other than the preshow of course, from 8 - 10 pm. I got tired of it after 4 or 5 trips and just sat on a bench watching them run by every few minutes. I think the posted wait time was 20 minutes.
Rock N Roller Coaster seems to hold a line later that the other rides though. About 3 trips in the last hour of operation is our average.