If you like the way an effect sounds mono, make it mono. If you want to create more three dimensionality (like adding life to a bland synth string track), then use them in stereo. Sometimes mono is easier to work with if you want to do things like auto-panning an effect from left to right, or precisely locate a delay (for instance) opposite from a dry source in the stereo field.
But I'm just speaking about effects in general. I'm not sure what you mean by a loop effect.