hi Skip,
Just my opinion here, so don't take this as advice or recommendation
If you're playing as one-man-band, then I would stay in stereo. The thing is, you're
a band which consists of drummer, bass guitarist, pianist, maybe strings, etc. and you definitely don't wish that all musicians would sound from single spot on the stage.
But because you already have stereo setup, you can try by yourself: simply use one speaker (connected to L/mono) and subwoofer. In this case keyboard will automatically down-mix from stereo to mono and so all voices will be present in that single speaker (+subwoofer).
What I'm saying is, listening to music that is recorded in mono (60+ years ago) is already bad on stereo equipment.. but listening mono music on single speaker only is simply not acceptable nowadays.
However, if you're playing in a real band, then I think mono is not so much of a trouble.. especially if you're mostly playing single instruments (i.e. piano or organ only).
Just my 2c worth,
Bogdan