No, I'm not saying that!
Her hair keeps changing, it's just that everything you've seen so far is also in conformance with regulations (except for the somewhat disheveled styles, which might be an artifact of gate travel).
Oh, and the military sometimes throws their grooming requirements out the window, such as in Afghanistan where a beard was required in certain branches so the natives wouldn't freak out, and the bushier the better, within reason.
I wish SG-1 showed more of her full-on smile. Often she grins on the show, but if you've seen her turn on a big beaming smile (like she often does in the director's specials and such), well, she goes all Christie Brinkley super-model radiant. Perhaps they decided that would detract from the serious nature of her character, though.