Yes, for EM symmetry, you must have geometric symmetry. In addition, you must also have excitation symmetry. For example, if there port on one side of the plane of symmetry, there must be a second port on the other side, and it must be excited symmetrically (+ or -) so that the two ports are treated as one port (push-pull, or push-push, see SonnetLite documentation for more detail if this is not clear). In addition, for time harmonic (i.e., RF/microwave) the surfaces about which you have symmetry must be planes (i.e., not lines, not cylinders, not spheres, etc.).