The trick is how to do it. Turn ON the tape machine. No harm will come if you don't set input, but the meters and possibly your monitors may object. The reason for input is when power is applied, the power supplies may not come up symmetrically and there is the possibility of a DC current pulse passing through the head. DC and iron make permanent magnets.
Magnets erase tape.
The reason no harm comes to the tape machine is that all that happens is the head amplifiers clip. As these are inherently low current amplifiers, there is no danger to the devices in the amplifiers. I have done this on almost all multitrack tape decks made in the last 20 years. The same technique works for cassette decks and open reel recorders. To actually demagnetize the heads, plug the demagger in at least four feet away from the heads and bring it up slowly to the heads.