I would recommend getting there at least 25 minutes early if only because of an experience we had when the only time I could get for breakfast was right at opening one day. We got to the CM who checks people in for pre-opening breakfast 20-25 minutes before our reservation and there where few groups in front of us checking in. We were one group away from checking in when the CM said that check in was now closed and we would have to get in a regular entrance line to go in. I told her that our reservation was AT opening (can't remember if that was 8:00 or 9:00). She said they don't do check in for times that are at park opening. There were people behind us that had reservations BEFORE park opening and she told them to get in line because they were opening the turnstiles. There were crying little princesses concerned they were going to miss seeing Cinderella. Our ADR was at BoG. So we ended up having to stand at the BACK of the shortest line we could find (which was STILL quite long) and of course, 50% of the people in front of us had problems with their magic bands (I'm sure it wasn't ACTUALLY that bad but I was frustrated). I was literally panicked as we power walked down Main Street and into Fantasyland. I was actually most concerned about being charged a no show fee when we had, in fact, shown up earlier than the 10 minute time they suggest. Luckily when we got to BoG (and had to wait in yet another line) and I explained what happened, it wasn't even a problem at all. But it was a terrible way to start the day.
That was only time I have EVER had an issue like that. We have even been let in early before when we had a reservation for 15 minutes AFTER the park opened (again I was paranoid we wouldn't be able to get there because we had someone in our party who could not walk quickly). If the policy is actually that you cannot get in early for a reservation right around opening I'm sure the restaurants are all very understanding if you can't get to the restaurant by your ADR time but I am a worrier!!
I should add that this happened prior to when they started allowing people onto Main Street. It wouldn't be as much of an issue if you didn't have to walk all the way from the park entrance.