More evidence she exists: In Exercise, he talks to her in Gene Stafford's office about bringing in Chinese food after seeing the remains of his last supper. Also in one other episode I can't remember the title of, he talks to her and she tells him to bring milk & a head of lettuce on his way home. And besides - where else would he get his pencils. She definitely exists!
Lest we forget the opening scene in Troubled Waters where her existence is snidely verified by Captain Gibbon.