My rule of thumb is if the instrument is mono it stays mono. Horns, vocals, woodwinds, etc always stay mono. A Piano or organ can go either mono or stereo depending on what the instrument is playing in the song. For instance if during the song a piano or organ have a solo then I have that track(s) stereo. If they are just background playing I would have them mono. Except if a leslie is added to any instrument then the track will be stereo. Large choruses are stereo as are ensembles, either vocal or orchestral.

But as always there are exceptions to every rule.

This is just my workflow and YMMV.

Whenever I get something stuck in the back of my throat, I dislodge it by drinking a beer.
It's called the Heineken Maneuver.

64 bit Win 10 Pro, the latest BiaB/RB, Roland Octa-Capture audio interface, a ton of software/hardware