It is necessary to use T-verts when the quad flow from one direction meets the quad flow from another direction.
On this rabbit model, an acceptable T-vertex forms where the edge flow up the side of the ear meets the edge flow across the top of the ear.
Clean Edge Flow
On cell phones and other electronics, an acceptable T-vertex forms when the quad flow from the side of the phone forms a corner with the flow across the bottom of the phone.
