Q. When should I use mono reverb as opposed to stereo reverb?

Another nifty trick for chart-targeted pop and EDM productions is to widen a reverb effect until it gives that ‘outside the speakers’ illusion, and then use just a small amount of it to expand the apparent width of your mix as a whole. Although such a reverb will have such dreadful mono-compatibility that it may pretty much vanish in mono, that’s rarely a great loss in practice, because the reverb serves no musical function. Better to lose some reverb in mono, than an important musical line!

Mike Senior, “Sound on Sound”