I have a paper copy, a photo on my phone, the QR code saved, and the Code written down. Boy Scout and ex-Military, always got at least a plan B.Living in the DR, I learned you should have Plan A, B and even C because at times things do go well. You can arrive at the airport and your internet is down or the eticket system is not working then the airline would not let you check in......big problem.
I suggest Plan A: 3 day before your departure, plan B: 2 days Before, Plan C: 1 day before. Have a good and relax trip.