Hi
It does not matter whether traffic goes through the MSFC and then to the FWSM or to the FWSM and then to the MSFC. Either way you can use the FWSM in routed or transparent mode.
if you do use transparent mode then no you would not need to define routing on the MSFC as you having one ip subnet.
In routed mode you would need routes on your MSFC via the outside interface of your FWSM.
HTH
Jon