mono or stereo interleave button

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.. 
 

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