I've read that a lot of people recommend mono for live performance.

Is this one of those recommendations you refer to?

In live settings, especially small venues, mono sound ensures that every member of the audience hears the same thing, no matter where they're standing. It prevents any issues that might arise from uneven speaker placement

Some things to consider are 1) the size of the venue you are playing in, 2) whether you are playing solo or with several other band members and 3) the capabilities, if any, of the venus's sound and mixing system.

I play out regularly with the CP88 and Montage 6 using stereo outputs and panning left right in the mains.

If you do that playing solo that is what the first quote refers to. A live performance in your living room may mean that some people are sitting much closer to the Left (or right) speaker than to the other speaker. They won't hear much of a stereo sound and what they do hear won't be very accurate.

Similarly if you are using effects like reverb. In a small venue people close to a wall will hear something different than people in the middle of a small room.

For that case the first quote says it all - mono will at least make sure everyone hears the same thing even though it means giving up some of that 'special sauce' (panning, stereo) you have created.

In larger venues that is less of a problem. And when a venue has their own sound/mixing system you would generally feed your stereo mix into their mixer and let their technician worry about making every instrument sound right for that venue.

What the above means is that, in some cases, you need to prepare a different setup that takes the venue issues into account. That could even mean duplicates sets of performances/voices configure for the venue. Then you don't have to do any manual configuration later.

Yamaha always, consistently, without fail ... recommends using stereo.  Mono is a compromise on the core sound without effects and on the effects themselves.   Searching the forum for discussions around this topic would find lots of Yamaha support messages advocating for stereo.

The only place I get requests for mono or the suggestion that "mono is better" is from the sound(wo)man.  Where having mono saves a channel on the board, a cable drop, and the overall complexity of the mix.  Since my shows are run on a budget with the budget rig and person to run it, I almost always run mono because it's "better" for the soundman.  Stereo vs mono is even something the audience would be able to tell the difference (and likely pick stereo every time) so it's fairly high on the recommendation list if you can swing it.

Since my shows are run on a budget with the budget rig and person to run it, I almost always run mono because it's "better" for the soundman.

Maybe you can address this then. Do you leave your performances 'as is' but just run one cable from the L/Mono jack? I seem to recall an article or thread where Bad Mister said something about the instrument doing the proper blending/balancing to make that work.

Yes.

I found one of Bad Mister's replies where he explained how the L/Mono jack is set up.

https://yamahasynth.com/community/modx-series-synthesizers/right-output-on-modx-isnt-as-loud-as-left-output/#post-86907

The link provides the full context but here is the relevant part

If (improperly) tested by moving a single cable from the left output to the right output, the left channel will seem louder than the right. This is completely normal.

This is an improper test because, the Left channel includes both left and right channels summed in mono when you use a single cable. You will see it is clearly labeled “L/Mono”. This means: in a situation where you are using this instrument with a mono sound system, a single cable connected to this jack will include a phase coherent sum of both Left and Right signal

The Right channel, when a cable is plugged in (assumes you are using both channels) only includes the right channel information. Using a single cable to the right channel is never correct.

The key phrase is that 'will include a phase coherent sum of both Left and Right signal'. That describes the L/Mono jack when only a single cable is used. When two cables are detected the L/Mono jack provides ONLY the Left channel and not that 'phase coherent sum' of both channels.