Rick Snoman talks a bit about this in the mixing theory section of The Dance Music Manual, basically saying that it can be helpful to use a combination of both mono and stereo sounds in a mix to help it maintain more definition and clarity. Some elements will naturally be mono (i.e. if you use a single mic to record vocals).. but it's good to be aware of the samples/plugins you are using.

It's not to say good mixes can't be made using all stereo sample sources. You can help yourself out quite a bit by panning things around, but the stereo width of the sounds will mean you will have more overlap then if you were to use a mono sample in its place. It's just to say that using some mono samples can be helpful in avoiding every element fighting each other on the "horizontal sound-stage" in the creation of a clear mix.