The question is in the title, but below are my thoughts on this.
As we know there are various possible attacks on printer, so it is not very secure to print high security things like private keys for bitcoin/gpg/etc. on random printer.
What I'm mostly concerned is hacked firmware and side-channel attacks, but maybe I'm looking at this wrong.
I don't worry about network because I can connect the printer directly to the computer that is not connected to any network.
I was looking for printer that is designed for security, maybe designed with open hardware, similarily to USB Armory.