The best way to use a stereo keyboard in mono is to sum the r and left outputs externally. Most keyboards (including Yamaha do this with internal circuitry) but also include mono pianos,

I don't hear the drastic differences in the mono outputs vs Stereo as Scott on the Tyros. But everything is subjective in the sound field.

Pro concert players are send the FOH a mono signal form their Motifs Whether summing AFTER they get a nice stereo mix for themselves or just using the left out.

The phase issues of the mono out are not worse than those same issues cropping up in a live room for those not sitting in the "sweet spot" and hearing reflections of separate program material. Big rooms never use Stereo for FOH sound systems yet they all have stereo keyboards and effects from guitar racks and efx boxes...go figure