Depends. Some VST3 plug-ins "idle" if there's no audio passing through them, but that ability has to be supported in the plug-in (no guarantees) and the host (SONAR does).
With VST2, I would assume there's at least a minimal hit with an FX in a bin...but it's something worth testing to find out exactly what happens. I don't have an FX in a bin unless I'm using it, so I've never really checked this out.