I've wondered this for quite a while.
 
When you have a mono wav on a track (say a voc), does it make any difference if the little mono/stereo button is mono or stereo?
I do get weirdness sometimes if I have varying (stereo/mono) plugs in the effects bin (some of the waves EQs) so I just keep the buttons all stereo and things seem to work out ok. I do this mainly because I sometimes loose track of whether or not the button is on mono or stereo even though it usually defaults to what the wav data is.
 
Thanks for any wisdom.. 
 

Often it doesn't matter. You're just splitting a single data stream into two identical streams, which is what that mono track is going to do anyway when it's sent on to a bus.
 
Where you run into trouble is sending stereo data to a mono plugin or vice versa. Some plugins handle this seamlessly, but others do not.

If you have a stereo style plug in on a mono track - such as a single guitar into a stereo reverb, you will want to enable the track itself to be stereo.  Sometimes I will use S Gear (amp sim) which can operate in stereo or mono - and it needs to see a stereo track even though the original audio date is mono

Thanks guys.