For a 9:00 opening be at the tapstiles no later than 8:20. Preferably a little earlier. You want to, no - have to, be as close to the front as you can. The closer you are to the 'front of the pack' the shorter you standby line will be.
In July we arrived at the park at 8:45 on a posted 9:00 opening. I knew we needed to be there earlier, but we blew it. They started letting people through the tapstiles at 8:45. We were through about 10 minutes behind the 'front of the pack', and our wait ended up being 45 minutes.
On Dec 30 we arrived at the park at 7:20 for a posted 8:00 opening. We were 4 people back from the tapstiles. They started letting people in at 7:40. We were at the front of the pack and we walked right on the ride, zero wait, first ride vehicle. As we exited the ride 10 minutes after we entered the posted standby time was 60 minutes.
Moral of the story? BE THERE EARLY. Seriously, 10 minutes behind the leaders can cost you 45 to 60 minutes or more. Yes, some people say you are just trading waits by sitting outside the tapstiles for 30 minutes. However, that 30 minutes up front can literally save you hours. On Dec 30th we went straight to RnR after TSMM. Waited 10 minutes, when we got off 10 minutes later the posted standby was over 60 minutes. Over to ToT where we waited another 10, and the standby time was again over a hour when we got off. So someone just 10 minutes behind us coming through the tapstiles would have had to spend over 3 hours in standby lines to ride TSMM, RnR and ToT, whereas we only spent 20 minutes. Yes, add the 20 minutes we were there before opening and we waited 40 minutes total, but by being there early we saved two and a half hours of waiting. Then we had FP+ in hand when we went to the MK that night!