QUESTION:
Were any plane change maneuvers needed for directing MGS towards Mars?

ANSWER from Charles Whetsel on November 30, 1997:
For the launch opportunities available for the MGS trajectory to Mars in the
1996 launch season, no deep-space plane change maneuvers were required. The
1992 trajectories used by the Mars Observer mission only required plane
changes for the later dates in the window.

All planetary launches attempt to launch at a time of day which does not
require a parking orbit plane-change (this is always possible for launch
sites whose latitude is less than the inclination of the earth's pole with
respect to the ecliptic).  Once a launch vehicle has been chosen (the Delta
rocket, for MGS and Pathfinder), there are ground facilities to support
those vehicles only at certain launch sites (Cape Canaveral and Vandenburg
Air Force Bases).