OK, we just got back from SWW and the weekends for this were completely crazy! If you do plan on doing it on Fri/Sat/Sun, I'm not sure what the *official* time is to sign up, but I would get there as early as possible and go straight there. Even if sign up is at 9 am, the line at opening will be horrible and you probably won't get in if you don't get in line and wait right away.
We got there one morning for SWW (can't remember if it was EMH or not) and went straight to the line and it was already crazy. I asked a CM if we would get into one of the shows for that day, considering where we already were in line, and she couldn't confirm yes or no. I didn't want to stand there if we weren't going to make it.
Just a tip: The same show is put on everyday, not just weekends, so if your trip falls on a day during the week, it shouldn't be as crazy. Only on weekends do you have to sign up first thing in the morning and within an hour all spots are gone! The CM told me to just go over to the stage at the posted times during the week and there shouldn't be any problems.
Also, if you do decide to go on the weekend, have someone else go pull fastpasses for TSM while you are in line to sign up.
I hope this makes sense and helps out a little
