I've been doing quite a bit of reading on this since I'm doing something similar to Ken. I've read that even some effects not listed as stereo ( analog chorus for example ) treat left and right differently, so unless steps are taken, using one xlr out could be missing some of the signal.
Also read about the noise gate with specific settings ( unity gain ) at the end of EACH PATCH, would sum to mono so one xlr out could be used. Also important to note that if using 1/4 out, set the line/guitar switch to LINE if doing direct to PA.
The other "big tip" I read was set input 1 to guitar and input 2 to variax for best sound if not actually using a variax -- not both inputs to guitar or same. ( and make that a global setting ).
These articles on using one xlr out also touch on the mixer in the effects chain, and cause me to glaze over. Unless I hear a lot of noise using 1/4 out I think I'll stick to that into direct box.