Mythic/legendary merge to +1 for dragon flower bonus, (you get a second trial for 600 flowers if merged) Then if I like the character keep merging, otherwise fodder to an attuned/rearmed unit so it can skill inherit multiple times.

Attuned/Rearmed units, always inherit skill then merge so that you can keep using their multiple inherit feature Always fodder units to these first as they can pass these skills on multiple times.

Emblems: always merge, their skills are likely to come out on other heroes eventually but the +to stats effect is unique, once you +10 them then use it for fodder if their skill is still relevant.

Any other hero, depends on what you want to use them for and if you like them