I’m a huge fan of simplicity, but there is precedent for hyphenated names in tags and it’s more accurate so latter-day_saints seems like the best possible tag in OSM, regardless of personal feelings about the tag or the organization. I don’t see many singular denominations in the wiki, they tend to be whatever the church name is minus “church of.” Since we map what’s on the ground, and people will see signs that say it’s a church of Latter-Day Saints – lowercase with underscores instead of spaces, it seems the most correct value is latter-day_saints.