I take pretty much the same route into work every day; the only thing that changes is which particular side roads I use to get from one arterial to another. This is limited by (1) topography, and (2) the fact that it's really, really difficult to get up in the mornings. My ride in is hilly, but direct ... and there really isn't a way around the hills, so direct is what matters.

While I've also found the ideal route home, I tend to vary this more. I don't usually have an immediate "deadline" after work, so I'll take different streets or trails if it's nice out, to change my scenery, or for whatever reason. Sometimes I'm just having too much fun to stop, and do a lap around the lake...