Cut iridised glass on the back
The idea seems to be to get a more even score and avoid chipping of the iridised surface.
First, the iridised surface is almost microscopic in thickness. It is put onto the surface as a mist of metallic oxides as it begins its run through the annealing lehr. This thickness will not affect your scoring.
The back is usually rougher side of the glass and so will be more difficult to get a smooth, even score than the front iridised surface.
Chipping of the iridised surface is caused by too much pressure during the scoring. Reduce your pressure and review your scoring practice.
Scoring the iridised surface with appropriate pressure will produce a clean break without chipping the surface. These comments apply to dichroic and flashed glass too.