If you really cannot print it, I would at least jot down the E-TKT number somewhere (start with 160). CX recently switched to a new reservation system, and there have been reports about people not being able to check-in because the agents cannot find their reservation / TKT. By the way, even if you have printed your E-TKT, you would still need to check-in (either online or at the counter) before the TSA will let you through security, so this should not be a "or" question.