The correct way is to turn on the bulk eraser, bring the tape up to it, erase and slowly move the reel away. Doing both sides gives an even field all across the tape and a more consistent background noise. Just to be on the safe side, set your wallet with credit cards down a few feet away from where you will be demagging. While it is true that some power amps in the past used to suffer from input coloration when the input attenuators were turned down, good modern amps usually do not suffer from this problem. Set the input level on the power amp to give a level slightly louder than you would ever listen when the control room monitor pot is fully up on the console and you will achieve maximum signal to noise ratio for the control room monitors.
All desks exhibit some noise.
Adding 40db or more typical power amp gain to the noise and attenuating the console output needlessly, results in less than optimum signal to noise ratio heard on the monitor loudspeakers. The residual console noise is perceived as a masking signal which degrades the transparency of the perceived mix. The mixer often adds more eq or compression than necessary to get the mix to jump out of the muck. Consequently on a playback system without the masking noise, the mix sounds overly processed and unpleasant to hear. The same principle applies to Cue systems.