We always recommend printing your e-ticket if you have the chance. If your phone runs out of battery or if you are unable to locate your e-ticket, you may be denied entry.
If you are not able to print your ticket, it is likely that you are able to show your tickets on your phone, but please check with the festival. If you plan on showing them on your phone, make sure that you have downloaded the ticket to your phone - as there is very often limited phone reception at large scale events. Just zoom in on the barcode when you arrive and ensure that your screen brightness is not too low.