It depends. I've done a half dozen MM's this year, and it seems to be ran a little different every time. Sometimes they start letting people through the turnstiles 15 minutes early, and then do a rope drop on Main St. Other times they don't even start checking tickets until 2 minutes after. I prefer the rope drop method, because it keeps you from getting stuck behind multiple groups that need their pass pictures taken etc.
I'm not really sure why the morning process varies as much as it does. Maybe different people in charge on different days? Who knows. Just get there 30 min prior to MM and you should be toward the front. And if you're going to Peter Pan, GO STRAIGHT THERE. That is the one ride in both parks that never has a slow time. Not even during MM/ EMH. Your only chance for a short wait on that one is to be one of the first there. The wait can easily be 25-30 min within about 5 minutes of opening.