Always depends on the source. Most of the time I sample mono (mix of the left and right channel) but some drums dont 'feel' right when you mix left and right together to a mono signal. In those cases I use only one side of the break. I realized that most of the time this is happen if the dry drums are placed in the middle and a reverb is more on one side than the other. I often sample the side with less reverb then. cause it seem to have more beef. As soon as the reverb side is mixed to it it loose this knocking and get a bit thin.
So the question for me is not "stereo or mono? "
but "stereo or mono mix of left and right or mono right or mono left"

peace