It depends more on the type of plug-in you are using. There are three types: mono in - mono out, mono in - stereo out, stereo in - stereo out. Plug ins with stereo outs require stereo FX tracks.
I don't use the term 'Fx channel but in Sonar in general all the paths are 'dual. If they carry mono they act (appear) as such. When you insert a stereo effect it passes the out as stereo, or look at an effect send -it has a pan. The exception is and 'interleave button that will force the channel (or insert) to stay mono.