Cassp... Mono is mono. Whether the channels are summed in the keyboard, or in the mixer SHOULD make no difference. If the keyboard has inferior electronics (summing buss prone to distortion) then maybe it would be better at the mixer, but in your E60's case, I wouldn't worry.

Plus, when using the mono out, you make sure that any EQ applied at the mixer is identical on both L & R, which if you use two channels of the mixer, you can't guarantee that. Any phase canceling problems (you probably already noticed the main piano sounds a little different in mono than it does in stereo) will be the same if the summing is internal or external, as well.

Save yourself a cable (and potentially, halve your chances of a bad one - tough to track down if you monitor in mono) and just use the mono out....