If you want to machine both sides of the part in the same program (machine one side, program stop, turn over part, continue machining) then yes, you will need a CS for the other side. When you create the CS make sure you toggle the depth axis in the correct direction, which is normally opposite the other CS.
You do not need to have the correct CS view active to drive the tool paths.
You do need to select the correct CS in the process tile before you drive your toolpath. This will be listed the the Mach CS box. The CS selected in this box will determine the correct orientation when you start selecting your geometry, faces, solids to drive toolpaths. If this is what you mean by making the CS plane active then you are correct.
This all assumes you are working with the Advanced Milling module |