Ian:
In the past I have cut  nuclear antigen control slides(er,pr ki-67) and frozen them at -80(unbaked) i bake after they defrosted and had them keep antigenicity for a good 6 months, also i take other controls like cytokeratins ((which lose antigenicity at a slower rate) and cut and store at 4 degrees. because of these observations i now store all control slides (s100, hmb45, vimentin , etc. at 4 degrees and usually only cut about 2 dozen in advance. hope this helps.
                          dana