NEVER! Because Yamaha's 'stereo sampled' voices rely on added 'effects' for their rich impressive sound, unfortunately, when mixed to mono, these effects are 'canceled out' due to the phenomenon of stereo ' phase cancellation ', resulting in a thin lifeless tinny sound of hearing only the raw sample itself.

Interestingly enough , this phase cancellation phenomenon appears to be most prominent (unique?) with Yamaha 'stereo sampled' voices, as I've not experienced 'phase cancellation ' when going out mono on Korg, Roland, or Ketron arrangers.

This article: Mono Comparability and Phase Relationships , may provide clues to why Yamaha's stereo samples suffer from stereo to mono phase cancellation problems. I hope Yamaha will look into this.

Scott
Scott Yee Entertainment