it's nice to be there early so that you can see the opening ceremonies (if there is one)..
when we were there recently, the opening ceremonies were as follows:
MK - they let people through the turnstiles but held us in the area before going under the train station....then exactly on the hour of opening, the mayor and a bunch of dancers came out to welcome us and dance for us....and then the train pulled into the station carrying Mickey and some other characters (as well as the family picked for the opening)...
after this little bit of welcome is done, they let you in....it's a very cute opening...
Epcot - they had us wait until opening hour...then a big bus came by carrying the characters (and a family came by in a replica test track car)...after the opening ceremony was over, we had to walk quickly behind a line of CMs leading the way to Soarin (or test track if you turned the other way - but we only headed for Soarin so i don't know what happens the otehr way)....the CMs lead you all the way into the land pavilion, to Soarin....so that no one can run..
AK.....we were let in before opening, but only up to the Tree of Life, where there was a rope....there we had to wait for opening hour when a bus came up with characters on it (Goofy, Minnie, pluto and maybe someone else)....they went through this cute thing and then said, "where's mickey"....who then can be seen in the distance in front of the tree of life...
when the opening is done they drop the rope and the CMs lead the pack (we follow the CMs to dinosaur, but there are of course CMs leading in the other direction to Kilimanjaro Safaris)....again to prevent people from running....but they do walk very fast...
MGM.....i'm trying to remember what they did here......they've changed this opening from how it used to be but i can't remember what they do here...
but they also open before opening and let you go down a ways to where a rope is.....at opening, after the opening ceremony (whatever it was, i can't remember), they drop the rope and again lead you at a fast walk to the big attractions (basically tower of terror and rock and roller coaster)...
so yes, it's nice to be there early so that you have an opportunity to be in position to see the ceremonies..