You may think everyone knows where north is because your entire work
group is more than familiar with the area in question. If your map’s
audience is your work group, then you are more than justified in omitting the north arrow. However, if there is an inkling of possibility that
someone from “the outside” will view the map, perhaps it wouldn’t
hurt to allow an unobtrusive, simple, north arrow as standard practice.
I caution, though, that you have to decide when standard practice verges
on the ridiculous. For example, a disclaimer on every map that your
department creates is a nice idea most of the time. But when you are
passing out directions to the ice cream store where you are holding a
staff meeting and are required to include a disclaimer that takes up half
the map, it could get silly.