Yeah I kind of wondered about that. That seems a rather odd motto for the family, given what we've seen.
Most of the house words work, but the "Hear Me Roar" seems rather meaningless for the family that regularly plots in dark rooms, assassinates and backstabs.
What I love about the Houses's words is that nearly all of them look like battle cries or threats:
Lannister: "Hear Me Roar"
Baratheon: "Ours is the Fury"
Targaryen: " Fire and Blood"
Bolton: "Our Blades are Sharp"
Greyjoy: "We Do Not Sow"
Mormont: "Here We Stand"
Or they can be some words that describe their personality and way of life:
Martell: "Unbent, Unbowed, Unbroken"
Tully: "Family,Duty, Honor"
Arryn: "As High As Honor"
Tyrell: "Growing Strong"
Tallhart: "Proud and Free"
Karstark: "The Sun of Winter"
Flint: "Ever Vigilant"
Redfort: "As Strong As Stone"
Tarly: "First In Battle"
Ok. You get my point. NEARLY all of the houses have "positive" words, whether they be battlecries, or descriptive.
The only one who makes the exception, by having grim, negative, warningfull words, is, you got it, House Stark.
"Winter is Coming" is practically a teaching. It teaches that life is not easy. And that one must be prepared for every adversity life will bring. And it is the only house motto that actually tells the truth: winter is coming/valar morghulis/ every man will die, whether one way or another.
Double apologises por the long post and terrible english