I believe that was my "rule" which I came up with after paddling with MagicPaddler one day.  I kept forgetting about him in back, being mostly a solo paddler, and would hop out upon landing without keeping any weight on the front of the canoe, nearly dumping him every time when the bow came up and the rear got tippy.  We were in an unloaded canoe.  I think it does pertain to loaded canoes too, because if your front paddler is light and the stern is around 200 pounds it still drops the rear of the canoe when the bowman gets out and immediately becomes much more tippy (IMO).  The stern paddler is sitting much closer to the back end of the canoe than the bow paddler is from the front end of the canoe, compounding the effect of un-weighting the bow first, as opposed to the rear first.  This issue might also be dependent on the kind of canoe used, my experiences have been with the deep keeled Winona Kevlar canoes.
As to it being a "rule", I agree with the rest that it all depends on the situation.  We usually try to land sideways to shore, so each paddler can get out on solid footing, rather than front end first.  This is often easy to do  if you are near a ledge rock of any size, picking an area with enough water to float the boat right up to the edge of the rock.  Sometimes it is not easy to do or is impractical to do it this way.   ;)