For smaller gigs I play my Yamaha P120 through either a single Roland KC 500 amp or the smaller (much lighter!) KC150. In either case I use a single lead going from the L/R output on the P120 into the amp. I then set the speakers on the piano to about half volume which provides a good monitor with the amp directed away from me. If there is a large p.a on site I'll go through that and that can give a great sound but I'm not sure what advantage stereo would bring (with my objective of getting as near to the sound of a good acoustic piano as possible)... I'm not so aware of the sound distinctly panning left>right when sat at a grand with the lid up, for example.
If you're playing through a keyboard amp, then clearly you'd send a mono signal, as keyboard amps tend to be a mono source. However, sending a stereo signal to a larger sound system can be beneficial, especially for concert work. How much you 'spread' the sound is a matter of taste!
I'm surprised you can't hear the stereo effect of playing a grand piano - there's a reasonable chance you'd hear more bass coming from the left hand side and more treble coming from the right, something to do with the physical placement of the strings, hammers, I understand....................