Most of us have heard these terms many times if we have practiced more than a couple years. This makes some sense.
All stances obviously involve tensions on both the insides and the outsides of the legs.
But considering the relationship between the two, particularly which set of muscles is dominant, does have some value. Perhaps there is. So let us examine zenkutsudachi a little closer. Back stance and side stance are much like the hanmi position of front stance, and probably involve some limited extra outside tension of the leg muscles. Side-stance may involve just a slight outward push on the knees past this natural point.