We have a big AM that is simulcast in two adjacent markets with FM translators. This is a spoken word formatted station with mostly ministries, not unlike Mike's station in Omaha, so no musical programming.
Both translators have been run in mono since I put them on the air. In both market, they are the only translators that are in mono and I think it helps substantially in making the signal more usable, especially outside of the 60 dBu contours. I am constantly sampling and comparing on a variety of radios, and for us, it's not even a close call. Making the decision for the receivers, in staying in mono all the time, is definitely worth it. If I had a Class C, I'd probably go stereo however.