I am sure the slice angle/orientation will bias the fibers present in LPP vs MPP of hippocampal slices.  However, you can reliably preserve both pathways simply using a perfect horizontal slice angle.  In my experience this was a very reasonable and reliable way to study evoked responses selectively in LPP vs. MPP.  You can even clearly see the inner, outer, and middle layers of the dentate gyrus molecular layer using standard DIC optics in live tissue slices. 

Thanks Jonathan.  I am interested in biasing fibers from the LPP/MPP using different slice orientations as I am electrically stimulating the fibers and recording from field potentials as a first pass sent of experiments.