Airlines let you scan that boarding pass barcode from your phone. HOWEVER, it is always a good idea to have a backup plan if you phone suddenly crashes/dies, battery runs down, internet at airport down, etc. Those kiosks where you check-in for your flight also let you print a boarding pass if you didn't happen to print one while at home. Storing a screen image on your phone is not a bad idea, but I prefer a printed backup that doesn't depend on my phone working.