In the very early days of electronic ticketing a large business travel agency in the UK had two senior employers of one of their (US) clients refused entry to the USA. Why? They had failed to give them the paper copy of the e-ticket. They had given an itinerary instead (which didn't have ticket numbers amongst other things). Since then I have always printed out the paper copy of my e-ticket and have had to show it (only a couple of times but show it I have).