In my opinion it really depends on the crowd level. While, I don’t think it fills up during EMM, I would go as soon as you can to sign up if it’s a must do.
My kids did Jedi training today. We were in line when the park opened. At 9:20 am it was our turn to choose times and 3pm was the first available. There was still a huge line waiting to pick times.
At 2:25 when we checked in, there were no times left and the cast member said that all alternate spots were full as well.
For the record today at HS it was insane. I have never seen so many people there before.