You can scan them in now. Two notes about doing it early:
1) if you want names for each ticket to show up in your app instead of the last 4 digits of the ticket number, you will need to add that as you scan them in. To keep the tickets straight, I also immediately wrote the assigned name on the back of the ticket. When we got to the turnstile, the CM wanted the actual paper tickets (not the scanned in ones) and I handed him one at a time and said, "This ticket is for this child." And he took the pic and moved on to the next ticket. This way the ticket number, nickname in my app, and the photo on record were all consistent.
2) When I added my tickets, the app said the tickets were good for 2 weeks. Don't worry!! This date will roll forward each day until you scan into the park the first day. Then that two week expiration date holds.
I'd also add a payment method to your account through the website (I don't think you can do this through the app, but not sure). And remember your Disney account password.
This is actually all very easy and fast. In fact, on the first morning, the CM scanned in the first two group's tickets before the gates even opened (during the countdown). I actually purchased MaxPass and booked our first FP before we were even through the main gates.