I ran this setup(SS in back / knobby up front) for a bit and really liked it. The rolling resistance of a semi-slick is so much lower than a knobby.
I rode last night with a new panaracer knobby on my back wheel for kicks and could certainly feel the difference. Almost felt mushy with 55lbs of pressure... didn't like it so much.
Only time I'd put a SS up front is if I were riding on a lot of hard pack.