Yes. MC deals less damage in a single tick than either of the other two.
On my sorc:
Creeping Terror (SF): 370 per tick
Affliction(WM): 447 per tick
Crushing Darkness (MC): 228 (274 when used on wrath proc)
Not sure how there is any doubt really?
Oh does the initial hit not get the increase? That would make sense then.
EDIT: Did some parsing and no it doesn't, learn something new every day.