The History of Galadriel and Celeborn

[Galin]

In note 8 to Of Dwarves And Men, Christopher Tolkien points to explicit statements in Appendix E (II) about the origin of the Runes, and comments about his father pondering something Elrond had said...

"... Thus the inconsistency, if inconsistency there was, could scarcely be removed; but in fact there was none. It was the "moon runes" that Elrond declared (...) to have been invented by the Dwarves and written by them with silver pens, not the Runes as an alphabetic form -- as my father at length noted with relief. I mention all this as an illustration of his intense concern to avoid discrepancy and inconsistency, even though in this case his anxiety was unfounded."

Christopher Tolkien, The Peoples of Middle-earth

Or try this: invent a "fantasy race" with a bit of a history and some details about them as they are "at present", and see how fast you are locked into a number of facts that you think should remain true as your tale unfolds. Doesn't mean you must abide by every detail of course, but how much will you overturn or refashion, and for what reason?

Concern with consistency is a general must; even when you choose to be inconsistent you are taking it into account, in a sense.