Mono/stereo instruments?

I am thinking, instruments like a piano, would be OK to have in mono or stereo, a choir seems more apropriate to have as stereo. A guitar would be mono. But instrument tracks are stereo or L or R? This are basic things but I dont really understand this. I use VST instruments mostly. How should I think about this, or should I not think, it works nicely to chose stereo when I add a instrument track?

Too many stereo signals in a mix can be difficult, so it can be helpful to reduce the stereo width of an instrument or even make it mono. As you already wrote, it really depends on the instrument, and of course the song/arrangement.
Most VST instruments have stereo outputs, even if they produce a more or less mono signal.
If you want to “monoize” that stereo signal, because it is too much, you can switch the panner in Cubase to “stereo balance panner” and use that, or simple insert the “stereo enhancer” plugin and hit the little “mono” button in that.
Sometimes with synthesizers, it is also the included effects that produce the stereo width (e.g. chorus, phaser etc). In that case, it can help to dial down those effects.

Great! Thanx! I thought my question was very silly, it seems to be an important issue then.