It's fairly easy to check whether your arranger collapses well to mono...

Plug into a stereo PA (or home system) using two mono channels of a stereo mixer. Play the stereo sounds with the pans set to wide apart, and listen carefully, then pan them both to 12 o'clock. Do you notice a substantial change in the timbre? Does it get 'pinched' or 'phase-y'?

If so, then there are issues.

You might also notice that reverbs, choruses, and many stereo modulation effects change sound substantially when collapsed. Not too much you can do about this, but if you regularly play on BOTH mono and stereo setups (at different venues, for instance), you might want to doublecheck your Registrations, and perhaps make a duplicate set optimized for mono, which often involves raising reverb levels, and perhaps changing to mono sounds if the internal stereo ones don't collapse gracefully...