I would always print at least two copies of everything (itins, boarding passes, hotel reservations, etc.), not only for backup but so that my wife would have a complete set of everything in case we separated (we had this incurable habit of volunteering for bumps ). However, I sure don't miss dealing with TSA's low-lifes -- my own life is so much more pleasant as a result.
I can already hear it: "This is a photocopy, not an original boarding pass. It's not valid." Unbelievable that we are still talking about how to protect ourselves from retaliation by these goons. Two years of bullying, harassment, robbing, and sexual assault. And still the numbskulls out there don't get it. As Mengele said, "The more we do to you, the less you seem to believe we are doing it."
Most probably aren't sufficiently socially advanced to understand that today's copies & printers are the same machines. I've always loved it when a ticket agent makes a big show out of getting my old boarding passes before she'll give me a new one ... while I have a full set of duplicates in my backpack or laptop case. And sometimes during irregular ops you have to insist on keeping the old ones to support your claim for original routing credit.