Looking at pictures of Titan rockets with solid rocket boosters, the bell is at an angle. Why would they do that?
As Ohsin comments above, the SRB nozzles are canted to direct the thrust approximately through the rocket's center of mass. This prevents any imbalance in thrust between the SRBs from creating a significant rotating torque -- instead, thrust imbalance will tend to make the rocket sideslip.
The downside of doing so is that some of the SRB thrust is wasted, the horizontal component of the booster's thrust being canceled out by the one on the opposite side. The vertical component of the thrust is given by the cosine of the cant angle (and so the effect is often called cosine loss). For a 6° cant, the vertical thrust is about 99.5% of the total thrust.