If you are batch sparging, where you put the slits doesn't matter much as long as the slits are on the floor.
If you are fly sparging, based on the location of that pipe it should have slits. The sites that say not to cut slits may be concerned about channeling if the "pipe" is closely spaced to other pipes or is along the wall. Ideally, the pipes are equally spaced and not against the wall so that the flow through the grain bed is even.
FYI, you should create a siphon to capture the wort below the outlet by having a hose extending below the floor attached to your outlet while draining. Longitudinal slits along the floor rather than cross-sectional slits theoretically will capture the most wort.