D-Nice states that so there is a consistent method and there can't be claims made against the results if they don't follow his method specifically. It's for people trying to get close to a D-Nice calibrated picture without him actually doing the calibration.
Now if you are simply wanting to age the phosphors for doing your own calibration or are having it calibrated, then there's no harm in watching stuff during the break in slides. You can even do it if you use his settings, but you can't expect to necessarily have the same results as following his method to the letter.
The real key - watch a wide variety of stuff that fills the screen and avoid static images. Whether you do that in conjunction with slides, on its own or just slides is up to you.
