perfectprint
i think it depends on the plugin. I know I have had some weird panning issues and then realized it is because I have a stereo plugin on a mono track.
Perfectprint, if you ever experience this, try changing your stereo interleave button to the opposite setting. Also, you may need to alter your pan law in Sonar's option. I haven't run into this issue you mention since older versions of Sonar.
For example, (and I don't know what versions these were) in the past, anytime you brought a stereo effect into Sonar's efx bin on a mono instrument, it would remain mono. You'd have to enable stereo interleave for the plug to literally send in stereo.
These days with the newer versions of Sonar, you don't have to touch the stereo interleave button unless you feel you need to. The plug goes stereo as soon as it's put in the bin even when set for mono interleave.
But in the past, I got a few weird issues with this and had to make sure that when I put a stereo plug on a mono track, that the interleave was set for stereo at all times. That said, you shouldn't be having that issue now and there should be no pan issues unless of course the plug you may use is older technology and your pan law is having a negative reaction to what you're using.
-Danny