If the app were more reliable generally, then your "maybe" is a good guess!
When we were there this past Saturday, even though I had the app open and ready when we passed through the tapstiles, as soon as I was in the park, my app went back to the log-in screen and asked for my password, which I foolishly didn't know and wasn't prepared for since I've never had this happen with MDX--it's just always on and open (Android, btw). All was okay, however, since my sister's app (iPhone) worked and she got the boarding group. I had to go to a CM to get my password reset, etc., and got locked out of MDX on my tablet later that day because it wouldn't recognize the new password!
I do think there's still an advantage to getting to the park early, because the lines to get in will be long and even if they don't open the boarding groups until the official park opening time, the people who get there earlier won't have to wait a long time just to tap in. I think people who really want to ride are going to show up early, no matter what. I know I would.
And, btw, RotR is totally 150% completely worth it. It's an amazing experience. I only wish we'd gone earlier in our trip so we could've done it more than twice. For those of you who haven't been yet--you're in for a HUGE treat. For those of us who've done it, you know what I'm talking about.