Missiles typically do not have a preferred plane or direction of maneuver and thus have symmetric airfoils.  Since the center of pressure for symmetric airfoils is relatively constant for small angle of attack, missile engineers typically speak of the complete center of pressure of the entire vehicle for stability and control analysis.  In missile analysis, the center of pressure is typically defined as the center of the additional pressure field due to a change in the angle of attack off of the trim angle of attack.
For unguided rockets the trim position is typically zero angle of attack and the center of pressure is defined to be the center of pressure of the resultant flow field on the entire vehicle resulting from a very small angle of attack (that is, the center of pressure is the limit as angle of attack goes to zero).  For positive stability in missiles, the total vehicle center of pressure defined as given above must be further from the nose of the vehicle than the center of gravity.  In missiles at lower angles of attack, the contributions to the center of pressure are dominated by the nose, wings, and fins.  The normalized normal force coefficient derivative with respect to the angle of attack of each component multiplied by the location of the center of pressure can be used to compute a centroid representing the total center of pressure.  The center of pressure of the added flow field is behind the center of gravity and the additional force "points" in the direction of the added angle of attack; this produces a moment that pushes the vehicle back to the trim position.
In guided missiles where the fins can be moved to trim the vehicles in different angles of attack, the center of pressure is the center of pressure of the flow field at that angle of attack for the undeflected fin position.  This is the center of pressure of any small change in the angle of attack (as defined above).  Once again for positive static stability, this definition of center of pressure requires that the center of pressure be further from the nose than the center of gravity.  This ensures that any increased forces resulting from increased angle of attack results in increased restoring moment to drive the missile back to the trimmed position.  In missile analysis, positive static margin implies that the complete vehicle makes a restoring moment for any angle of attack from the trim position.