Well I think that your problem is not physics, your problem is engineering. To get a precise angle the answer depends on the inertia tensor of that certain body. You would have to solve newton equation of motion for a rigid body or, in a more aerospatial spirit, tell the ship to add a torque strong enough to stop when it reaches the desired position. In any case you would need pen and paper but I bet second option is easier. I guess that real space ships would have the same problem as you do, and in fact satellites have to use gyroscopes to rotate and then they use it in the opposite way to stop (because of the small friction in space).

Physics is not intended to allow precise positioning. To rotate a rigidbody with torque, you should write a PID controller, but feedback about the current rigidbody orientation is very unreliable (eulerAngles may return weird values, and changes from 0-360 and 360-0 when near to the origin). Take a look at this question, where I proposed an alternative physics control.

EDITED: The general idea behind a PID controller is explained in this pseudo code:

var pGain: float; // adjust the individual gains
var iGain: float; // iGain better to be 0 in this case
var dGain: float;
private var lastPError: float = 0;

function PidLoop(){
  var pError = desiredAngle - currentAngle; // calculate the proportional error...
  var iError += pError * Time.deltaTime; // the integral error...
  var dError = (pError - lastPError)/ Time.deltaTime; // and the differential error
  lastPError = pError;
  // the torque is the sum of all errors weighted by their gains:
  torque = pGain*pError+iGain*iError+dGain*dError;
  // apply the torque to the object
  ApplyTorque(torque);
}

The function PidLoop must be called at a constant rate, like in Unity's FixedUpdate. currentAngle is the current object angle about the axis of interest (Y, for instance), and desiredAngle is the angle you want to reach. pError is the proportional error: it's just the difference between what you want and the present position. iError is the integral error, a numeric integration of the proportional error, and dError is the differential error, a numeric differentiation of pError. The 3 errors are summed with individual weights, and the resulting value is applied as torque about the same axis currentAngle is measured.
If only pError were used, the final position would never be reached because the force applied is proportional to the error - thus zero error means zero force. To cancel this error while still mantaining some control force, the iError is used: any small pError accumulates in iError over time, creating a force that will eventualy cancel any opposite forces. If there are no opposite forces (as usually happens in rotational PID), the corresponding iGain must be set to zero, or the system will become very unstable. The differential error, dError, is very important in rotational PID, because it becomes negative when the desired angle is getting closer, acting as a break and slowing down the rotation.
In Unity, the big problem is how to get a reliable feedback signal - the controller gets crazy when around 0, or if the other axes are non-zero. Maybe in your 2D engine this error is easier to work around - at least there are no other axes to worry about. Try to adapt an algorithm like this to your case.

You have to assign a rigidbody component to the object and add torque, check this out:

http://unity3d.com/support/documentation/ScriptReference/Rigidbody.AddTorque.html