you can use one output, and it won't matter which, unless you have stereo effects set up.
You also could set it Master Mono...
Yes, and you should IF you have stereo effects selected.
Otherwise it's not worth the bother, and one less thing to remember to change or change back.
Even giving the foh stereo-outs, theres always a chance he/she may mix to mono.
Id make sure all your sounds are equally balanced L &R then it shouldn't matter if they take left or right or both mix to mono or stereo.
I’ve found some sound guys have a default instant-apply LPF & HPF, assuming all guitars require this, perhaps the standard for ill-sounding “traditional” amp rigs.
As for being stoned, shove some tabasco sauce down his bong 
If you set main outs to Master Mono you don’t even have to bother checking for any FX stereo patches. The KPa will sum those to mono and send a mono signal to both outs so your sound guy can plug into either and everything will be fine.
I really recommend setting the Main Out to Master Mono instead.
As for being stoned, shove some tabasco sauce down his bong
Tried it. Doesn't work if you like spicy food……
